diff --git a/github/github-accessors.go b/github/github-accessors.go
index 6378b40b44c..307d6e339d3 100644
--- a/github/github-accessors.go
+++ b/github/github-accessors.go
@@ -25942,6 +25942,30 @@ func (s *StatusEvent) GetUpdatedAt() Timestamp {
 	return *s.UpdatedAt
 }
 
+// GetAfterID returns the AfterID field if it's non-nil, zero value otherwise.
+func (s *SubIssueRequest) GetAfterID() int64 {
+	if s == nil || s.AfterID == nil {
+		return 0
+	}
+	return *s.AfterID
+}
+
+// GetBeforeID returns the BeforeID field if it's non-nil, zero value otherwise.
+func (s *SubIssueRequest) GetBeforeID() int64 {
+	if s == nil || s.BeforeID == nil {
+		return 0
+	}
+	return *s.BeforeID
+}
+
+// GetReplaceParent returns the ReplaceParent field if it's non-nil, zero value otherwise.
+func (s *SubIssueRequest) GetReplaceParent() bool {
+	if s == nil || s.ReplaceParent == nil {
+		return false
+	}
+	return *s.ReplaceParent
+}
+
 // GetCreatedAt returns the CreatedAt field if it's non-nil, zero value otherwise.
 func (s *Subscription) GetCreatedAt() Timestamp {
 	if s == nil || s.CreatedAt == nil {
diff --git a/github/github-accessors_test.go b/github/github-accessors_test.go
index 3588b7909f1..f2a51a6028c 100644
--- a/github/github-accessors_test.go
+++ b/github/github-accessors_test.go
@@ -33332,6 +33332,39 @@ func TestStatusEvent_GetUpdatedAt(tt *testing.T) {
 	s.GetUpdatedAt()
 }
 
+func TestSubIssueRequest_GetAfterID(tt *testing.T) {
+	tt.Parallel()
+	var zeroValue int64
+	s := &SubIssueRequest{AfterID: &zeroValue}
+	s.GetAfterID()
+	s = &SubIssueRequest{}
+	s.GetAfterID()
+	s = nil
+	s.GetAfterID()
+}
+
+func TestSubIssueRequest_GetBeforeID(tt *testing.T) {
+	tt.Parallel()
+	var zeroValue int64
+	s := &SubIssueRequest{BeforeID: &zeroValue}
+	s.GetBeforeID()
+	s = &SubIssueRequest{}
+	s.GetBeforeID()
+	s = nil
+	s.GetBeforeID()
+}
+
+func TestSubIssueRequest_GetReplaceParent(tt *testing.T) {
+	tt.Parallel()
+	var zeroValue bool
+	s := &SubIssueRequest{ReplaceParent: &zeroValue}
+	s.GetReplaceParent()
+	s = &SubIssueRequest{}
+	s.GetReplaceParent()
+	s = nil
+	s.GetReplaceParent()
+}
+
 func TestSubscription_GetCreatedAt(tt *testing.T) {
 	tt.Parallel()
 	var zeroValue Timestamp
diff --git a/github/github.go b/github/github.go
index a3b1941eac1..c366820383d 100644
--- a/github/github.go
+++ b/github/github.go
@@ -222,6 +222,7 @@ type Client struct {
 	Search             *SearchService
 	SecretScanning     *SecretScanningService
 	SecurityAdvisories *SecurityAdvisoriesService
+	SubIssue           *SubIssueService
 	Teams              *TeamsService
 	Users              *UsersService
 }
@@ -458,6 +459,7 @@ func (c *Client) initialize() {
 	c.Search = (*SearchService)(&c.common)
 	c.SecretScanning = (*SecretScanningService)(&c.common)
 	c.SecurityAdvisories = (*SecurityAdvisoriesService)(&c.common)
+	c.SubIssue = (*SubIssueService)(&c.common)
 	c.Teams = (*TeamsService)(&c.common)
 	c.Users = (*UsersService)(&c.common)
 }
diff --git a/github/strings_test.go b/github/strings_test.go
index a164cebcac0..98906b4d41d 100644
--- a/github/strings_test.go
+++ b/github/strings_test.go
@@ -107,6 +107,7 @@ func TestString(t *testing.T) {
 		{Hook{ID: Ptr(int64(1))}, `github.Hook{ID:1}`},
 		{IssueComment{ID: Ptr(int64(1))}, `github.IssueComment{ID:1}`},
 		{Issue{Number: Ptr(1)}, `github.Issue{Number:1}`},
+		{SubIssue{ID: Ptr(int64(1))}, `github.SubIssue{ID:1}`},
 		{Key{ID: Ptr(int64(1))}, `github.Key{ID:1}`},
 		{Label{ID: Ptr(int64(1)), Name: Ptr("l")}, `github.Label{ID:1, Name:"l"}`},
 		{Organization{ID: Ptr(int64(1))}, `github.Organization{ID:1}`},
diff --git a/github/sub_issue.go b/github/sub_issue.go
new file mode 100644
index 00000000000..8b24adb899b
--- /dev/null
+++ b/github/sub_issue.go
@@ -0,0 +1,140 @@
+// Copyright 2025 The go-github AUTHORS. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package github
+
+import (
+	"context"
+	"fmt"
+)
+
+// SubIssueService handles communication with the sub-issue related
+// methods of the GitHub API.
+//
+// Sub-issues help you group and manage your issues with a parent/child relationship.
+//
+// GitHub API docs: https://docs.github.com/rest/issues/sub-issues
+type SubIssueService service
+
+// SubIssue represents a GitHub sub-issue on a repository.
+// Note: As far as the GitHub API is concerned, every pull request is an issue,
+// but not every issue is a pull request. Some endpoints, events, and webhooks
+// may also return pull requests via this struct. If PullRequestLinks is nil,
+// this is an issue, and if PullRequestLinks is not nil, this is a pull request.
+// The IsPullRequest helper method can be used to check that.
+type SubIssue Issue
+
+func (i SubIssue) String() string {
+	return Stringify(i)
+}
+
+// SubIssueListByIssueOptions specifies the optional parameters to the
+// SubIssueService.ListByIssue method.
+type SubIssueListByIssueOptions struct {
+	IssueListByRepoOptions
+}
+
+// SubIssueRequest represents a request to add, remove, or reprioritize sub-issues.
+type SubIssueRequest struct {
+	SubIssueID    int64  `json:"sub_issue_id"`             // Required: The ID of the sub-issue
+	AfterID       *int64 `json:"after_id,omitempty"`       // Optional: Position after this sub-issue ID
+	BeforeID      *int64 `json:"before_id,omitempty"`      // Optional: Position before this sub-issue ID
+	ReplaceParent *bool  `json:"replace_parent,omitempty"` // Optional: Whether to replace the existing parent
+}
+
+// Remove a sub-issue from the specified repository.
+//
+// GitHub API docs: https://docs.github.com/rest/issues/sub-issues#remove-sub-issue
+//
+//meta:operation DELETE /repos/{owner}/{repo}/issues/{issue_number}/sub_issue
+func (s *SubIssueService) Remove(ctx context.Context, owner, repo string, subIssueNumber int64, subIssue SubIssueRequest) (*SubIssue, *Response, error) {
+	u := fmt.Sprintf("repos/%v/%v/issues/%v/sub_issues", owner, repo, subIssueNumber)
+
+	req, err := s.client.NewRequest("DELETE", u, subIssue)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	si := new(SubIssue)
+	resp, err := s.client.Do(ctx, req, si)
+	if err != nil {
+		return nil, resp, err
+	}
+
+	return si, resp, nil
+}
+
+// ListByIssue lists all sub-issues for the specified issue.
+//
+// GitHub API docs: https://docs.github.com/rest/issues/sub-issues#list-sub-issues
+//
+//meta:operation GET /repos/{owner}/{repo}/issues/{issue_number}/sub_issues
+func (s *SubIssueService) ListByIssue(ctx context.Context, owner, repo string, issueNumber int64, opts *IssueListOptions) ([]*SubIssue, *Response, error) {
+	u := fmt.Sprintf("repos/%v/%v/issues/%v/sub_issues", owner, repo, issueNumber)
+	u, err := addOptions(u, opts)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	req, err := s.client.NewRequest("GET", u, nil)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	var subIssues []*SubIssue
+	resp, err := s.client.Do(ctx, req, &subIssues)
+	if err != nil {
+		return nil, resp, err
+	}
+
+	return subIssues, resp, nil
+}
+
+// Add adds a sub-issue to the specified issue.
+//
+// The sub-issue to be added must belong to the same repository owner as the parent issue.
+// To replace the existing parent of a sub-issue, set replaceParent to true.
+//
+// GitHub API docs: https://docs.github.com/rest/issues/sub-issues#add-sub-issue
+//
+//meta:operation POST /repos/{owner}/{repo}/issues/{issue_number}/sub_issues
+func (s *SubIssueService) Add(ctx context.Context, owner, repo string, issueNumber int64, subIssue SubIssueRequest) (*SubIssue, *Response, error) {
+	u := fmt.Sprintf("repos/%v/%v/issues/%v/sub_issues", owner, repo, issueNumber)
+	req, err := s.client.NewRequest("POST", u, subIssue)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	si := new(SubIssue)
+	resp, err := s.client.Do(ctx, req, si)
+	if err != nil {
+		return nil, resp, err
+	}
+
+	return si, resp, nil
+}
+
+// Reprioritize changes a sub-issue's priority to a different position in the parent list.
+//
+// Either afterId or beforeId must be specified to determine the new position of the sub-issue.
+//
+// GitHub API docs: https://docs.github.com/rest/issues/sub-issues#reprioritize-sub-issue
+//
+//meta:operation PATCH /repos/{owner}/{repo}/issues/{issue_number}/sub_issues/priority
+func (s *SubIssueService) Reprioritize(ctx context.Context, owner, repo string, issueNumber int64, subIssue SubIssueRequest) (*SubIssue, *Response, error) {
+	u := fmt.Sprintf("repos/%v/%v/issues/%v/sub_issues/priority", owner, repo, issueNumber)
+	req, err := s.client.NewRequest("PATCH", u, subIssue)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	si := new(SubIssue)
+	resp, err := s.client.Do(ctx, req, si)
+	if err != nil {
+		return nil, resp, err
+	}
+
+	return si, resp, nil
+}
diff --git a/github/sub_issue_test.go b/github/sub_issue_test.go
new file mode 100644
index 00000000000..9a88b2441d0
--- /dev/null
+++ b/github/sub_issue_test.go
@@ -0,0 +1,173 @@
+// Copyright 2025 The go-github AUTHORS. All rights reserved.
+//
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package github
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+)
+
+func TestSubIssuesService_Add(t *testing.T) {
+	t.Parallel()
+	client, mux, _ := setup(t)
+
+	input := &SubIssueRequest{SubIssueID: 42}
+
+	mux.HandleFunc("/repos/o/r/issues/1/sub_issues", func(w http.ResponseWriter, r *http.Request) {
+		v := new(SubIssueRequest)
+		assertNilError(t, json.NewDecoder(r.Body).Decode(v))
+
+		testMethod(t, r, "POST")
+		if !cmp.Equal(v, input) {
+			t.Errorf("Request body = %+v, want %+v", v, input)
+		}
+
+		fmt.Fprint(w, `{"id":42, "number":1}`)
+	})
+
+	ctx := context.Background()
+	got, _, err := client.SubIssue.Add(ctx, "o", "r", 1, *input)
+	if err != nil {
+		t.Errorf("SubIssues.Add returned error: %v", err)
+	}
+
+	want := &SubIssue{Number: Ptr(1), ID: Int64(42)}
+	if !cmp.Equal(got, want) {
+		t.Errorf("SubIssues.Add = %+v, want %+v", got, want)
+	}
+
+	const methodName = "Add"
+	testNewRequestAndDoFailure(t, methodName, client, func() (*Response, error) {
+		got, resp, err := client.SubIssue.Add(ctx, "o", "r", 1, *input)
+		if got != nil {
+			t.Errorf("testNewRequestAndDoFailure %v = %#v, want nil", methodName, got)
+		}
+		return resp, err
+	})
+}
+
+func TestSubIssuesService_ListByIssue(t *testing.T) {
+	t.Parallel()
+	client, mux, _ := setup(t)
+
+	mux.HandleFunc("/repos/o/r/issues/1/sub_issues", func(w http.ResponseWriter, r *http.Request) {
+		testMethod(t, r, "GET")
+
+		fmt.Fprint(w, `[{"id":1},{"id":2}]`)
+	})
+
+	ctx := context.Background()
+	opt := &IssueListOptions{}
+	issues, _, err := client.SubIssue.ListByIssue(ctx, "o", "r", 1, opt)
+	if err != nil {
+		t.Errorf("SubIssues.ListByIssue returned error: %v", err)
+	}
+
+	want := []*SubIssue{{ID: Int64(1)}, {ID: Int64(2)}}
+	if !cmp.Equal(issues, want) {
+		t.Errorf("SubIssues.ListByIssue = %+v, want %+v", issues, want)
+	}
+
+	const methodName = "ListByIssue"
+	testBadOptions(t, methodName, func() (err error) {
+		_, _, err = client.SubIssue.ListByIssue(ctx, "\n", "\n", 1, opt)
+		return err
+	})
+
+	testNewRequestAndDoFailure(t, methodName, client, func() (*Response, error) {
+		got, resp, err := client.SubIssue.ListByIssue(ctx, "o", "r", 1, opt)
+		if got != nil {
+			t.Errorf("testNewRequestAndDoFailure %v = %#v, want nil", methodName, got)
+		}
+		return resp, err
+	})
+}
+
+func TestSubIssuesService_Remove(t *testing.T) {
+	t.Parallel()
+	client, mux, _ := setup(t)
+
+	input := &SubIssueRequest{SubIssueID: 42}
+
+	mux.HandleFunc("/repos/o/r/issues/1/sub_issues", func(w http.ResponseWriter, r *http.Request) {
+		v := new(SubIssueRequest)
+		assertNilError(t, json.NewDecoder(r.Body).Decode(v))
+
+		testMethod(t, r, "DELETE")
+		if !cmp.Equal(v, input) {
+			t.Errorf("Request body = %+v, want %+v", v, input)
+		}
+
+		fmt.Fprint(w, `{"id":42, "number":1}`)
+	})
+
+	ctx := context.Background()
+	got, _, err := client.SubIssue.Remove(ctx, "o", "r", 1, *input)
+	if err != nil {
+		t.Errorf("SubIssues.Remove returned error: %v", err)
+	}
+
+	want := &SubIssue{ID: Int64(42), Number: Ptr(1)}
+	if !cmp.Equal(got, want) {
+		t.Errorf("SubIssues.Remove = %+v, want %+v", got, want)
+	}
+
+	const methodName = "Remove"
+	testNewRequestAndDoFailure(t, methodName, client, func() (*Response, error) {
+		got, resp, err := client.SubIssue.Remove(ctx, "o", "r", 1, *input)
+		if got != nil {
+			t.Errorf("testNewRequestAndDoFailure %v = %#v, want nil", methodName, got)
+		}
+		return resp, err
+	})
+}
+
+func TestSubIssuesService_Reprioritize(t *testing.T) {
+	t.Parallel()
+	client, mux, _ := setup(t)
+
+	input := &SubIssueRequest{SubIssueID: 42, AfterID: Int64(5)}
+
+	mux.HandleFunc("/repos/o/r/issues/1/sub_issues/priority", func(w http.ResponseWriter, r *http.Request) {
+		testMethod(t, r, "PATCH")
+
+		v := new(SubIssueRequest)
+		assertNilError(t, json.NewDecoder(r.Body).Decode(v))
+
+		testMethod(t, r, "PATCH")
+		if !cmp.Equal(v, input) {
+			t.Errorf("Request body = %+v, want %+v", v, input)
+		}
+
+		fmt.Fprint(w, `{"id":42, "number":1}`)
+	})
+
+	ctx := context.Background()
+	got, _, err := client.SubIssue.Reprioritize(ctx, "o", "r", 1, *input)
+	if err != nil {
+		t.Errorf("SubIssues.Reprioritize returned error: %v", err)
+	}
+
+	want := &SubIssue{ID: Int64(42), Number: Ptr(1)}
+	if !cmp.Equal(got, want) {
+		t.Errorf("SubIssues.Reprioritize = %+v, want %+v", got, want)
+	}
+
+	const methodName = "Reprioritize"
+
+	testNewRequestAndDoFailure(t, methodName, client, func() (*Response, error) {
+		got, resp, err := client.SubIssue.Reprioritize(ctx, "o", "r", 1, *input)
+		if got != nil {
+			t.Errorf("testNewRequestAndDoFailure %v = %#v, want nil", methodName, got)
+		}
+		return resp, err
+	})
+}