diff --git a/internal/backend/remote-state/s3/backend.go b/internal/backend/remote-state/s3/backend.go
index 2474f19df1b2..042f5f6c144c 100644
--- a/internal/backend/remote-state/s3/backend.go
+++ b/internal/backend/remote-state/s3/backend.go
@@ -386,6 +386,7 @@ func (b *Backend) PrepareConfig(obj cty.Value) (cty.Value, tfdiags.Diagnostics)
 			Validators: []stringValidator{
 				validateStringNotEmpty,
 				validateStringS3Path,
+				validateStringDoesNotContain("//"),
 			},
 		}
 		keyValidators.ValidateAttr(val, attrPath, &diags)
diff --git a/internal/backend/remote-state/s3/backend_test.go b/internal/backend/remote-state/s3/backend_test.go
index 228ba3e407fa..21cbc4e171b1 100644
--- a/internal/backend/remote-state/s3/backend_test.go
+++ b/internal/backend/remote-state/s3/backend_test.go
@@ -1137,6 +1137,20 @@ func TestBackendConfig_PrepareConfigValidation(t *testing.T) {
 				),
 			},
 		},
+		"key with double slash": {
+			config: cty.ObjectVal(map[string]cty.Value{
+				"bucket": cty.StringVal("test"),
+				"key":    cty.StringVal("test/with/double//slash"),
+				"region": cty.StringVal("us-west-2"),
+			}),
+			expectedDiags: tfdiags.Diagnostics{
+				attributeErrDiag(
+					"Invalid Value",
+					`Value must not contain "//"`,
+					cty.GetAttrPath("key"),
+				),
+			},
+		},
 
 		"null region": {
 			config: cty.ObjectVal(map[string]cty.Value{
diff --git a/internal/backend/remote-state/s3/validate.go b/internal/backend/remote-state/s3/validate.go
index 00b0ae1a47d5..c094f1fc7c12 100644
--- a/internal/backend/remote-state/s3/validate.go
+++ b/internal/backend/remote-state/s3/validate.go
@@ -121,6 +121,18 @@ func validateStringMatches(re *regexp.Regexp, description string) stringValidato
 	}
 }
 
+func validateStringDoesNotContain(s string) stringValidator {
+	return func(val string, path cty.Path, diags *tfdiags.Diagnostics) {
+		if strings.Contains(val, s) {
+			*diags = diags.Append(attributeErrDiag(
+				"Invalid Value",
+				fmt.Sprintf(`Value must not contain "%s"`, s),
+				path,
+			))
+		}
+	}
+}
+
 func validateStringInSlice(sl []string) stringValidator {
 	return func(val string, path cty.Path, diags *tfdiags.Diagnostics) {
 		match := false
diff --git a/internal/backend/remote-state/s3/validate_test.go b/internal/backend/remote-state/s3/validate_test.go
index b947d11aa799..4dad4f838cb8 100644
--- a/internal/backend/remote-state/s3/validate_test.go
+++ b/internal/backend/remote-state/s3/validate_test.go
@@ -751,3 +751,46 @@ func TestValidateStringURL(t *testing.T) {
 	}
 
 }
+
+func Test_validateStringDoesNotContain(t *testing.T) {
+	t.Parallel()
+
+	path := cty.GetAttrPath("field")
+
+	testcases := map[string]struct {
+		val      string
+		s        string
+		expected tfdiags.Diagnostics
+	}{
+		"valid": {
+			val: "foo",
+			s:   "bar",
+		},
+
+		"invalid": {
+			val: "foobarbaz",
+			s:   "bar",
+			expected: tfdiags.Diagnostics{
+				attributeErrDiag(
+					"Invalid Value",
+					`Value must not contain "bar"`,
+					path,
+				),
+			},
+		},
+	}
+
+	for name, testcase := range testcases {
+		testcase := testcase
+		t.Run(name, func(t *testing.T) {
+			t.Parallel()
+
+			var diags tfdiags.Diagnostics
+			validateStringDoesNotContain(testcase.s)(testcase.val, path, &diags)
+
+			if diff := cmp.Diff(diags, testcase.expected, cmp.Comparer(diagnosticComparer)); diff != "" {
+				t.Errorf("unexpected diagnostics difference: %s", diff)
+			}
+		})
+	}
+}