diff --git a/README.md b/README.md
index 2e60e92..4842e5b 100644
--- a/README.md
+++ b/README.md
@@ -189,15 +189,6 @@ By default, literal matching is performed. The assertion fails if
 }
 ```
 
-The expected output can be specified with a heredoc or standard input as well.
-
-```bash
-@test 'assert_output() with pipe' {
-  run echo 'have'
-  echo 'want' | assert_output
-}
-```
-
 On failure, the expected and actual output are displayed.
 
 ```
@@ -266,6 +257,23 @@ invalid.
 This option and partial matching (`--partial` or `-p`) are mutually
 exclusive. An error is displayed when used simultaneously.
 
+#### Standard Input, HereDocs and HereStrings
+
+The expected output can be specified via standard input (also
+heredoc/herestring) with the `-`/`--stdin` option.
+
+```bash
+@test 'assert_output() with pipe' {
+  run echo 'hello'
+  echo 'hello' | assert_output -
+}
+
+@test 'assert_output() with herestring' {
+  run echo 'hello'
+  assert_output - <<< hello
+}
+```
+
 
 ### `refute_output`
 
@@ -287,15 +295,6 @@ By default, literal matching is performed. The assertion fails if
 }
 ```
 
--The unexpected output can be specified with a heredoc or standard input as well.
-
-```bash
-@test 'refute_output() with pipe' {
-  run echo 'want'
-  echo 'want' | refute_output
-}
-```
-
 On failure, the output is displayed.
 
 ```
@@ -363,6 +362,23 @@ invalid.
 This option and partial matching (`--partial` or `-p`) are mutually
 exclusive. An error is displayed when used simultaneously.
 
+#### Standard Input, HereDocs and HereStrings
+
+The unexpected output can be specified via standard input (also
+heredoc/herestring) with the `-`/`--stdin` option.
+
+```bash
+@test 'refute_output() with pipe' {
+  run echo 'hello'
+  echo 'world' | refute_output -
+}
+
+@test 'refute_output() with herestring' {
+  run echo 'hello'
+  refute_output - <<< world
+}
+```
+
 
 ### `assert_line`
 
diff --git a/src/assert.bash b/src/assert.bash
index 1194753..1aac74d 100644
--- a/src/assert.bash
+++ b/src/assert.bash
@@ -172,8 +172,9 @@ assert_failure() {
 # Options:
 #   -p, --partial - partial matching
 #   -e, --regexp - extended regular expression matching
+#   -, --stdin - read expected output from the standard input
 # Arguments:
-#   $1 - [=STDIN] expected output
+#   $1 - expected output
 # Returns:
 #   0 - expected matches the actual output
 #   1 - otherwise
@@ -185,12 +186,14 @@ assert_failure() {
 assert_output() {
   local -i is_mode_partial=0
   local -i is_mode_regexp=0
+  local -i use_stdin=0
 
   # Handle options.
   while (( $# > 0 )); do
     case "$1" in
       -p|--partial) is_mode_partial=1; shift ;;
       -e|--regexp) is_mode_regexp=1; shift ;;
+      -|--stdin) use_stdin=1; shift ;;
       --) shift; break ;;
       *) break ;;
     esac
@@ -205,7 +208,11 @@ assert_output() {
 
   # Arguments.
   local expected
-  (( $# == 0 )) && expected="$(cat -)" || expected="$1"
+  if (( use_stdin )); then
+    expected="$(cat -)"
+  else
+    expected="$1"
+  fi
 
   # Matching.
   if (( is_mode_regexp )); then
@@ -213,9 +220,7 @@ assert_output() {
       echo "Invalid extended regular expression: \`$expected'" \
         | batslib_decorate 'ERROR: assert_output' \
         | fail
-      return $?
-    fi
-    if ! [[ $output =~ $expected ]]; then
+    elif ! [[ $output =~ $expected ]]; then
       batslib_print_kv_single_or_multi 6 \
           'regexp'  "$expected" \
           'output' "$output" \
@@ -265,8 +270,9 @@ assert_output() {
 # Options:
 #   -p, --partial - partial matching
 #   -e, --regexp - extended regular expression matching
+#   -, --stdin - read unexpected output from the standard input
 # Arguments:
-#   $1 - [=STDIN] unexpected output
+#   $1 - unexpected output
 # Returns:
 #   0 - unexpected matches the actual output
 #   1 - otherwise
@@ -278,12 +284,14 @@ assert_output() {
 refute_output() {
   local -i is_mode_partial=0
   local -i is_mode_regexp=0
+  local -i use_stdin=0
 
   # Handle options.
   while (( $# > 0 )); do
     case "$1" in
       -p|--partial) is_mode_partial=1; shift ;;
       -e|--regexp) is_mode_regexp=1; shift ;;
+      -|--stdin) use_stdin=1; shift ;;
       --) shift; break ;;
       *) break ;;
     esac
@@ -298,7 +306,11 @@ refute_output() {
 
   # Arguments.
   local unexpected
-  (( $# == 0 )) && unexpected="$(cat -)" || unexpected="$1"
+  if (( use_stdin )); then
+    unexpected="$(cat -)"
+  else
+    unexpected="$1"
+  fi
 
   if (( is_mode_regexp == 1 )) && [[ '' =~ $unexpected ]] || (( $? == 2 )); then
     echo "Invalid extended regular expression: \`$unexpected'" \
diff --git a/test/50-assert-15-assert_output.bats b/test/50-assert-15-assert_output.bats
index cca79cc..90c2056 100755
--- a/test/50-assert-15-assert_output.bats
+++ b/test/50-assert-15-assert_output.bats
@@ -26,12 +26,20 @@ load test_helper
   [ "${lines[3]}" == '--' ]
 }
 
-@test 'assert_output(): reads <expected> from STDIN' {
+@test 'assert_output() - : reads <expected> from STDIN' {
   run echo 'a'
-  run assert_output <<STDIN
+  run assert_output - <<STDIN
+a
+STDIN
+  [ "$status" -eq 0 ]
+  [ "${#lines[@]}" -eq 0 ]
+}
+
+@test 'assert_output() --stdin : reads <expected> from STDIN' {
+  run echo 'a'
+  run assert_output --stdin <<STDIN
 a
 STDIN
-echo "$output"
   [ "$status" -eq 0 ]
   [ "${#lines[@]}" -eq 0 ]
 }
diff --git a/test/50-assert-16-refute_output.bats b/test/50-assert-16-refute_output.bats
index 5204301..05f2995 100755
--- a/test/50-assert-16-refute_output.bats
+++ b/test/50-assert-16-refute_output.bats
@@ -25,9 +25,18 @@ load test_helper
   [ "${lines[2]}" == '--' ]
 }
 
-@test 'refute_output(): reads <unexpected> from STDIN' {
-  run echo 'a'
-  run refute_output <<INPUT
+@test 'refute_output() - : reads <unexpected> from STDIN' {
+  run echo '-'
+  run refute_output - <<INPUT
+b
+INPUT
+  [ "$status" -eq 0 ]
+  [ "${#lines[@]}" -eq 0 ]
+}
+
+@test 'refute_output() --stdin : reads <unexpected> from STDIN' {
+  run echo '--stdin'
+  run refute_output --stdin <<INPUT
 b
 INPUT
   [ "$status" -eq 0 ]