diff --git a/src/encoding/json/decode_test.go b/src/encoding/json/decode_test.go
index 255ff5c66a75bf..04fbd7524dee2f 100644
--- a/src/encoding/json/decode_test.go
+++ b/src/encoding/json/decode_test.go
@@ -738,6 +738,18 @@ var unmarshalTests = []unmarshalTest{
 		out:    []intWithPtrMarshalText{1, 2, 3},
 		golden: true,
 	},
+
+	{in: `0.000001`, ptr: new(float64), out: 0.000001, golden: true},
+	{in: `1e-7`, ptr: new(float64), out: 1e-7, golden: true},
+	{in: `100000000000000000000`, ptr: new(float64), out: 100000000000000000000.0, golden: true},
+	{in: `1e+21`, ptr: new(float64), out: 1e21, golden: true},
+	{in: `-0.000001`, ptr: new(float64), out: -0.000001, golden: true},
+	{in: `-1e-7`, ptr: new(float64), out: -1e-7, golden: true},
+	{in: `-100000000000000000000`, ptr: new(float64), out: -100000000000000000000.0, golden: true},
+	{in: `-1e+21`, ptr: new(float64), out: -1e21, golden: true},
+	{in: `999999999999999900000`, ptr: new(float64), out: 999999999999999900000.0, golden: true},
+	{in: `9007199254740992`, ptr: new(float64), out: 9007199254740992.0, golden: true},
+	{in: `9007199254740993`, ptr: new(float64), out: 9007199254740992.0, golden: false},
 }
 
 func TestMarshal(t *testing.T) {
diff --git a/src/encoding/json/encode.go b/src/encoding/json/encode.go
index aab912601a542c..6e43a9d48b3054 100644
--- a/src/encoding/json/encode.go
+++ b/src/encoding/json/encode.go
@@ -526,7 +526,31 @@ func (bits floatEncoder) encode(e *encodeState, v reflect.Value, opts encOpts) {
 	if math.IsInf(f, 0) || math.IsNaN(f) {
 		e.error(&UnsupportedValueError{v, strconv.FormatFloat(f, 'g', -1, int(bits))})
 	}
-	b := strconv.AppendFloat(e.scratch[:0], f, 'g', -1, int(bits))
+
+	// Convert as if by ES6 number to string conversion.
+	// This matches most other JSON generators.
+	// See golang.org/issue/6384 and golang.org/issue/14135.
+	// Like fmt %g, but the exponent cutoffs are different
+	// and exponents themselves are not padded to two digits.
+	b := e.scratch[:0]
+	abs := math.Abs(f)
+	fmt := byte('f')
+	// Note: Must use float32 comparisons for underlying float32 value to get precise cutoffs right.
+	if abs != 0 {
+		if bits == 64 && (abs < 1e-6 || abs >= 1e21) || bits == 32 && (float32(abs) < 1e-6 || float32(abs) >= 1e21) {
+			fmt = 'e'
+		}
+	}
+	b = strconv.AppendFloat(b, f, fmt, -1, int(bits))
+	if fmt == 'e' {
+		// clean up e-09 to e-9
+		n := len(b)
+		if n >= 4 && b[n-4] == 'e' && b[n-3] == '-' && b[n-2] == '0' {
+			b[n-2] = b[n-1]
+			b = b[:n-1]
+		}
+	}
+
 	if opts.quoted {
 		e.WriteByte('"')
 	}
diff --git a/src/encoding/json/encode_test.go b/src/encoding/json/encode_test.go
index b484022a70e198..13e765afa31d1a 100644
--- a/src/encoding/json/encode_test.go
+++ b/src/encoding/json/encode_test.go
@@ -7,8 +7,11 @@ package json
 import (
 	"bytes"
 	"fmt"
+	"log"
 	"math"
 	"reflect"
+	"regexp"
+	"strconv"
 	"testing"
 	"unicode"
 )
@@ -611,3 +614,106 @@ func TestTextMarshalerMapKeysAreSorted(t *testing.T) {
 		t.Errorf("Marshal map with text.Marshaler keys: got %#q, want %#q", b, want)
 	}
 }
+
+var re = regexp.MustCompile
+
+// syntactic checks on form of marshalled floating point numbers.
+var badFloatREs = []*regexp.Regexp{
+	re(`p`),                     // no binary exponential notation
+	re(`^\+`),                   // no leading + sign
+	re(`^-?0[^.]`),              // no unnecessary leading zeros
+	re(`^-?\.`),                 // leading zero required before decimal point
+	re(`\.(e|$)`),               // no trailing decimal
+	re(`\.[0-9]+0(e|$)`),        // no trailing zero in fraction
+	re(`^-?(0|[0-9]{2,})\..*e`), // exponential notation must have normalized mantissa
+	re(`e[0-9]`),                // positive exponent must be signed
+	re(`e[+-]0`),                // exponent must not have leading zeros
+	re(`e-[1-6]$`),              // not tiny enough for exponential notation
+	re(`e+(.|1.|20)$`),          // not big enough for exponential notation
+	re(`^-?0\.0000000`),         // too tiny, should use exponential notation
+	re(`^-?[0-9]{22}`),          // too big, should use exponential notation
+	re(`[1-9][0-9]{16}[1-9]`),   // too many significant digits in integer
+	re(`[1-9][0-9.]{17}[1-9]`),  // too many significant digits in decimal
+	// below here for float32 only
+	re(`[1-9][0-9]{8}[1-9]`),  // too many significant digits in integer
+	re(`[1-9][0-9.]{9}[1-9]`), // too many significant digits in decimal
+}
+
+func TestMarshalFloat(t *testing.T) {
+	nfail := 0
+	test := func(f float64, bits int) {
+		vf := interface{}(f)
+		if bits == 32 {
+			f = float64(float32(f)) // round
+			vf = float32(f)
+		}
+		bout, err := Marshal(vf)
+		if err != nil {
+			t.Errorf("Marshal(%T(%g)): %v", vf, vf, err)
+			nfail++
+			return
+		}
+		out := string(bout)
+
+		// result must convert back to the same float
+		g, err := strconv.ParseFloat(out, bits)
+		if err != nil {
+			t.Errorf("Marshal(%T(%g)) = %q, cannot parse back: %v", vf, vf, out, err)
+			nfail++
+			return
+		}
+		if f != g || fmt.Sprint(f) != fmt.Sprint(g) { // fmt.Sprint handles ±0
+			t.Errorf("Marshal(%T(%g)) = %q (is %g, not %g)", vf, vf, out, float32(g), vf)
+			nfail++
+			return
+		}
+
+		bad := badFloatREs
+		if bits == 64 {
+			bad = bad[:len(bad)-2]
+		}
+		for _, re := range bad {
+			if re.MatchString(out) {
+				t.Errorf("Marshal(%T(%g)) = %q, must not match /%s/", vf, vf, out, re)
+				nfail++
+				return
+			}
+		}
+	}
+
+	var (
+		bigger  = math.Inf(+1)
+		smaller = math.Inf(-1)
+	)
+
+	var digits = "1.2345678901234567890123"
+	for i := len(digits); i >= 2; i-- {
+		for exp := -30; exp <= 30; exp++ {
+			for _, sign := range "+-" {
+				for bits := 32; bits <= 64; bits += 32 {
+					s := fmt.Sprintf("%c%se%d", sign, digits[:i], exp)
+					f, err := strconv.ParseFloat(s, bits)
+					if err != nil {
+						log.Fatal(err)
+					}
+					next := math.Nextafter
+					if bits == 32 {
+						next = func(g, h float64) float64 {
+							return float64(math.Nextafter32(float32(g), float32(h)))
+						}
+					}
+					test(f, bits)
+					test(next(f, bigger), bits)
+					test(next(f, smaller), bits)
+					if nfail > 50 {
+						t.Fatalf("stopping test early")
+					}
+				}
+			}
+		}
+	}
+	test(0, 64)
+	test(math.Copysign(0, -1), 64)
+	test(0, 32)
+	test(math.Copysign(0, -1), 32)
+}