diff --git a/src/net/textproto/reader.go b/src/net/textproto/reader.go
index 8e800088c1fe79..48ae2946a6b0e8 100644
--- a/src/net/textproto/reader.go
+++ b/src/net/textproto/reader.go
@@ -489,8 +489,11 @@ func readMIMEHeader(r *Reader, lim int64) (MIMEHeader, error) {
 	// large one ahead of time which we'll cut up into smaller
 	// slices. If this isn't big enough later, we allocate small ones.
 	var strs []string
-	hint := r.upcomingHeaderNewlines()
+	hint := r.upcomingHeaderKeys()
 	if hint > 0 {
+		if hint > 1000 {
+			hint = 1000 // set a cap to avoid overallocation
+		}
 		strs = make([]string, hint)
 	}
 
@@ -581,9 +584,9 @@ func mustHaveFieldNameColon(line []byte) error {
 
 var nl = []byte("\n")
 
-// upcomingHeaderNewlines returns an approximation of the number of newlines
+// upcomingHeaderKeys returns an approximation of the number of keys
 // that will be in this header. If it gets confused, it returns 0.
-func (r *Reader) upcomingHeaderNewlines() (n int) {
+func (r *Reader) upcomingHeaderKeys() (n int) {
 	// Try to determine the 'hint' size.
 	r.R.Peek(1) // force a buffer load if empty
 	s := r.R.Buffered()
@@ -591,7 +594,20 @@ func (r *Reader) upcomingHeaderNewlines() (n int) {
 		return
 	}
 	peek, _ := r.R.Peek(s)
-	return bytes.Count(peek, nl)
+	for len(peek) > 0 && n < 1000 {
+		var line []byte
+		line, peek, _ = bytes.Cut(peek, nl)
+		if len(line) == 0 || (len(line) == 1 && line[0] == '\r') {
+			// Blank line separating headers from the body.
+			break
+		}
+		if line[0] == ' ' || line[0] == '\t' {
+			// Folded continuation of the previous line.
+			continue
+		}
+		n++
+	}
+	return n
 }
 
 // CanonicalMIMEHeaderKey returns the canonical format of the
diff --git a/src/net/textproto/reader_test.go b/src/net/textproto/reader_test.go
index 9618b874e6f853..696ae406f3860f 100644
--- a/src/net/textproto/reader_test.go
+++ b/src/net/textproto/reader_test.go
@@ -10,6 +10,7 @@ import (
 	"io"
 	"net"
 	"reflect"
+	"runtime"
 	"strings"
 	"sync"
 	"testing"
@@ -129,6 +130,42 @@ func TestReadMIMEHeaderSingle(t *testing.T) {
 	}
 }
 
+// TestReaderUpcomingHeaderKeys is testing an internal function, but it's very
+// difficult to test well via the external API.
+func TestReaderUpcomingHeaderKeys(t *testing.T) {
+	for _, test := range []struct {
+		input string
+		want  int
+	}{{
+		input: "",
+		want:  0,
+	}, {
+		input: "A: v",
+		want:  1,
+	}, {
+		input: "A: v\r\nB: v\r\n",
+		want:  2,
+	}, {
+		input: "A: v\nB: v\n",
+		want:  2,
+	}, {
+		input: "A: v\r\n  continued\r\n  still continued\r\nB: v\r\n\r\n",
+		want:  2,
+	}, {
+		input: "A: v\r\n\r\nB: v\r\nC: v\r\n",
+		want:  1,
+	}, {
+		input: "A: v" + strings.Repeat("\n", 1000),
+		want:  1,
+	}} {
+		r := reader(test.input)
+		got := r.upcomingHeaderKeys()
+		if test.want != got {
+			t.Fatalf("upcomingHeaderKeys(%q): %v; want %v", test.input, got, test.want)
+		}
+	}
+}
+
 func TestReadMIMEHeaderNoKey(t *testing.T) {
 	r := reader(": bar\ntest-1: 1\n\n")
 	m, err := r.ReadMIMEHeader()
@@ -271,6 +308,28 @@ func TestReadMIMEHeaderTrimContinued(t *testing.T) {
 	}
 }
 
+// Test that reading a header doesn't overallocate. Issue 58975.
+func TestReadMIMEHeaderAllocations(t *testing.T) {
+	var totalAlloc uint64
+	const count = 200
+	for i := 0; i < count; i++ {
+		r := reader("A: b\r\n\r\n" + strings.Repeat("\n", 4096))
+		var m1, m2 runtime.MemStats
+		runtime.ReadMemStats(&m1)
+		_, err := r.ReadMIMEHeader()
+		if err != nil {
+			t.Fatalf("ReadMIMEHeader: %v", err)
+		}
+		runtime.ReadMemStats(&m2)
+		totalAlloc += m2.TotalAlloc - m1.TotalAlloc
+	}
+	// 32k is large and we actually allocate substantially less,
+	// but prior to the fix for #58975 we allocated ~400k in this case.
+	if got, want := totalAlloc/count, uint64(32768); got > want {
+		t.Fatalf("ReadMIMEHeader allocated %v bytes, want < %v", got, want)
+	}
+}
+
 type readResponseTest struct {
 	in       string
 	inCode   int