diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5ea2768..dc05c04 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -9,6 +9,7 @@
 - Source paths: Use `-P dir1:dir2` or a `source-path=dir1` directive
                 to specify search paths for sourced files.
 - json1 format like --format=json but treats tabs as single characters
+- Recognize FLAGS variables created by the shflags library.
 - SC2154: Also warn about unassigned uppercase variables (optional)
 - SC2252: Warn about `[ $a != x ] || [ $a != y ]`, similar to SC2055
 - SC2251: Inform about ineffectual ! in front of commands
diff --git a/src/ShellCheck/Analytics.hs b/src/ShellCheck/Analytics.hs
index f89c51f..37d97ab 100644
--- a/src/ShellCheck/Analytics.hs
+++ b/src/ShellCheck/Analytics.hs
@@ -2085,6 +2085,8 @@ prop_checkUnused38= verifyTree checkUnusedAssignments "(( a=42 ))"
 prop_checkUnused39= verifyNotTree checkUnusedAssignments "declare -x -f foo"
 prop_checkUnused40= verifyNotTree checkUnusedAssignments "arr=(1 2); num=2; echo \"${arr[@]:num}\""
 prop_checkUnused41= verifyNotTree checkUnusedAssignments "@test 'foo' {\ntrue\n}\n"
+prop_checkUnused42= verifyNotTree checkUnusedAssignments "DEFINE_string foo '' ''; echo \"${FLAGS_foo}\""
+prop_checkUnused43= verifyTree checkUnusedAssignments "DEFINE_string foo '' ''"
 checkUnusedAssignments params t = execWriter (mapM_ warnFor unused)
   where
     flow = variableFlow params
diff --git a/src/ShellCheck/AnalyzerLib.hs b/src/ShellCheck/AnalyzerLib.hs
index 388f871..508b6ee 100644
--- a/src/ShellCheck/AnalyzerLib.hs
+++ b/src/ShellCheck/AnalyzerLib.hs
@@ -606,6 +606,11 @@ getModifiedVariableCommand base@(T_SimpleCommand _ _ (T_NormalWord _ (T_Literal
         "mapfile" -> maybeToList $ getMapfileArray base rest
         "readarray" -> maybeToList $ getMapfileArray base rest
 
+        "DEFINE_boolean" -> maybeToList $ getFlagVariable rest
+        "DEFINE_float" -> maybeToList $ getFlagVariable rest
+        "DEFINE_integer" -> maybeToList $ getFlagVariable rest
+        "DEFINE_string" -> maybeToList $ getFlagVariable rest
+
         _ -> []
   where
     flags = map snd $ getAllFlags base
@@ -679,6 +684,12 @@ getModifiedVariableCommand base@(T_SimpleCommand _ _ (T_NormalWord _ (T_Literal
         map (getLiteralArray . snd)
             (filter (\(x,_) -> getLiteralString x == Just "-a") (zip (args) (tail args)))
 
+    -- get the FLAGS_ variable created by a shflags DEFINE_ call
+    getFlagVariable (n:v:_) = return (base, n, flagName n, DataString $ SourceFrom [v])
+      where
+        flagName varName@(T_NormalWord _ _) = "FLAGS_" ++ (onlyLiteralString varName)
+    getFlagVariable _ = fail "Invalid flag definition"
+
 getModifiedVariableCommand _ = []
 
 getIndexReferences s = fromMaybe [] $ do
diff --git a/src/ShellCheck/Data.hs b/src/ShellCheck/Data.hs
index 2eedeeb..1394c04 100644
--- a/src/ShellCheck/Data.hs
+++ b/src/ShellCheck/Data.hs
@@ -36,6 +36,11 @@ internalVariables = [
 
     -- Ksh
     , ".sh.version"
+
+    -- shflags
+    , "FLAGS_ARGC", "FLAGS_ARGV", "FLAGS_ERROR", "FLAGS_FALSE", "FLAGS_HELP",
+    "FLAGS_PARENT", "FLAGS_RESERVED", "FLAGS_TRUE", "FLAGS_VERSION",
+    "flags_error", "flags_return"
   ]
 
 specialVariablesWithoutSpaces = [
@@ -45,6 +50,9 @@ variablesWithoutSpaces = specialVariablesWithoutSpaces ++ [
     "BASHPID", "BASH_ARGC", "BASH_LINENO", "BASH_SUBSHELL", "EUID", "LINENO",
     "OPTIND", "PPID", "RANDOM", "SECONDS", "SHELLOPTS", "SHLVL", "UID",
     "COLUMNS", "HISTFILESIZE", "HISTSIZE", "LINES"
+
+    -- shflags
+    , "FLAGS_ERROR", "FLAGS_FALSE", "FLAGS_TRUE"
   ]
 
 specialVariables = specialVariablesWithoutSpaces ++ ["@", "*"]