diff --git a/changelog.md b/changelog.md
index 4005dc1230c9..493df6198893 100644
--- a/changelog.md
+++ b/changelog.md
@@ -138,6 +138,9 @@ provided by the operating system.
   (instead of skipping them sometimes as it was before).
 - Added optional `followSymlinks` argument to `setFilePermissions`.
 
+- Added `os.isAdmin` to tell whether the caller's process is a member of the
+  Administrators local group (on Windows) or a root (on POSIX).
+
 - Added `random.initRand()` overload with no argument which uses the current time as a seed.
 
 - Added experimental `linenoise.readLineStatus` to get line and status (e.g. ctrl-D or ctrl-C).
diff --git a/lib/pure/os.nim b/lib/pure/os.nim
index f62d23b6dbcb..3cf197570ef0 100644
--- a/lib/pure/os.nim
+++ b/lib/pure/os.nim
@@ -1648,6 +1648,36 @@ proc setFilePermissions*(filename: string, permissions: set[FilePermission],
       var res2 = setFileAttributesA(filename, res)
     if res2 == - 1'i32: raiseOSError(osLastError(), $(filename, permissions))
 
+proc isAdmin*: bool {.noWeirdTarget.} =
+  ## Returns whether the caller's process is a member of the Administrators local
+  ## group (on Windows) or a root (on POSIX), via `geteuid() == 0`.
+  when defined(windows):
+    # Rewrite of the example from Microsoft Docs:
+    # https://docs.microsoft.com/en-us/windows/win32/api/securitybaseapi/nf-securitybaseapi-checktokenmembership#examples
+    # and corresponding PostgreSQL function:
+    # https://doxygen.postgresql.org/win32security_8c.html#ae6b61e106fa5d6c5d077a9d14ee80569
+    var ntAuthority = SID_IDENTIFIER_AUTHORITY(value: SECURITY_NT_AUTHORITY)
+    var administratorsGroup: PSID
+    if not isSuccess(allocateAndInitializeSid(addr ntAuthority,
+                                              BYTE(2),
+                                              SECURITY_BUILTIN_DOMAIN_RID,
+                                              DOMAIN_ALIAS_RID_ADMINS,
+                                              0, 0, 0, 0, 0, 0,
+                                              addr administratorsGroup)):
+      raiseOSError(osLastError(), "could not get SID for Administrators group")
+
+    defer:
+      if freeSid(administratorsGroup) != nil:
+        raiseOSError(osLastError(), "failed to free SID for Administrators group")
+
+    var b: WINBOOL
+    if not isSuccess(checkTokenMembership(0, administratorsGroup, addr b)):
+      raiseOSError(osLastError(), "could not check access token membership")
+
+    return isSuccess(b)
+  else:
+    return geteuid() == 0
+
 proc createSymlink*(src, dest: string) {.noWeirdTarget.} =
   ## Create a symbolic link at `dest` which points to the item specified
   ## by `src`. On most operating systems, will fail if a link already exists.
diff --git a/lib/windows/winlean.nim b/lib/windows/winlean.nim
index 1334a85d549d..e65749407015 100644
--- a/lib/windows/winlean.nim
+++ b/lib/windows/winlean.nim
@@ -25,6 +25,7 @@ when useWinUnicode:
 else:
   type WinChar* = char
 
+# See https://docs.microsoft.com/en-us/windows/win32/winprog/windows-data-types
 type
   Handle* = int
   LONG* = int32
@@ -33,6 +34,7 @@ type
   WINBOOL* = int32
     ## `WINBOOL` uses opposite convention as posix, !=0 meaning success.
     # xxx this should be distinct int32, distinct would make code less error prone
+  PBOOL* = ptr WINBOOL
   DWORD* = int32
   PDWORD* = ptr DWORD
   LPINT* = ptr int32
@@ -40,6 +42,7 @@ type
   PULONG_PTR* = ptr uint
   HDC* = Handle
   HGLRC* = Handle
+  BYTE* = cuchar
 
   SECURITY_ATTRIBUTES* {.final, pure.} = object
     nLength*: int32
@@ -136,6 +139,10 @@ const
 
   HANDLE_FLAG_INHERIT* = 0x00000001'i32
 
+proc isSuccess*(a: WINBOOL): bool {.inline.} =
+  ## Returns true if `a != 0`. Windows uses a different convention than POSIX,
+  ## where `a == 0` is commonly used on success.
+  a != 0
 proc getVersionExW*(lpVersionInfo: ptr OSVERSIONINFO): WINBOOL {.
     stdcall, dynlib: "kernel32", importc: "GetVersionExW", sideEffect.}
 proc getVersionExA*(lpVersionInfo: ptr OSVERSIONINFO): WINBOOL {.
@@ -1129,5 +1136,42 @@ proc setFileTime*(hFile: Handle, lpCreationTime: LPFILETIME,
                  lpLastAccessTime: LPFILETIME, lpLastWriteTime: LPFILETIME): WINBOOL
      {.stdcall, dynlib: "kernel32", importc: "SetFileTime".}
 
+type
+  # https://docs.microsoft.com/en-us/windows/win32/api/winnt/ns-winnt-sid_identifier_authority
+  SID_IDENTIFIER_AUTHORITY* {.importc, header: "<windows.h>".} = object
+    value* {.importc: "Value"}: array[6, BYTE]
+  # https://docs.microsoft.com/en-us/windows/win32/api/winnt/ns-winnt-sid
+  SID* {.importc, header: "<windows.h>".} = object
+    Revision: BYTE
+    SubAuthorityCount: BYTE
+    IdentifierAuthority: SID_IDENTIFIER_AUTHORITY
+    SubAuthority: ptr ptr DWORD
+  PSID* = ptr SID
+
+const
+  # https://docs.microsoft.com/en-us/windows/win32/secauthz/sid-components
+  # https://github.com/mirror/mingw-w64/blob/84c950bdab7c999ace49fe8383856be77f88c4a8/mingw-w64-headers/include/winnt.h#L2994
+  SECURITY_NT_AUTHORITY* = [BYTE(0), BYTE(0), BYTE(0), BYTE(0), BYTE(0), BYTE(5)]
+  SECURITY_BUILTIN_DOMAIN_RID* = 32
+  DOMAIN_ALIAS_RID_ADMINS* = 544
+
+proc allocateAndInitializeSid*(pIdentifierAuthority: ptr SID_IDENTIFIER_AUTHORITY,
+                               nSubAuthorityCount: BYTE,
+                               nSubAuthority0: DWORD,
+                               nSubAuthority1: DWORD,
+                               nSubAuthority2: DWORD,
+                               nSubAuthority3: DWORD,
+                               nSubAuthority4: DWORD,
+                               nSubAuthority5: DWORD,
+                               nSubAuthority6: DWORD,
+                               nSubAuthority7: DWORD,
+                               pSid: ptr PSID): WINBOOL
+     {.stdcall, dynlib: "Advapi32", importc: "AllocateAndInitializeSid".}
+proc checkTokenMembership*(tokenHandle: Handle, sidToCheck: PSID,
+                           isMember: PBOOL): WINBOOL
+     {.stdcall, dynlib: "Advapi32", importc: "CheckTokenMembership".}
+proc freeSid*(pSid: PSID): PSID
+     {.stdcall, dynlib: "Advapi32", importc: "FreeSid".}
+
 when defined(nimHasStyleChecks):
   {.pop.} # {.push styleChecks: off.}
diff --git a/tests/stdlib/tos.nim b/tests/stdlib/tos.nim
index b47412a62798..3348bd6bbdc1 100644
--- a/tests/stdlib/tos.nim
+++ b/tests/stdlib/tos.nim
@@ -651,3 +651,10 @@ block: # normalizeExe
     doAssert "foo/../bar".dup(normalizeExe) == "foo/../bar"
   when defined(windows):
     doAssert "foo".dup(normalizeExe) == "foo"
+
+block: # isAdmin
+  let isAzure = existsEnv("TF_BUILD") # xxx factor with testament.specs.isAzure
+  # In Azure on Windows tests run as an admin user
+  if isAzure and defined(windows): doAssert isAdmin()
+  # In Azure on POSIX tests run as a normal user
+  if isAzure and defined(posix): doAssert not isAdmin()