diff --git a/Marlin/Configuration_adv.h b/Marlin/Configuration_adv.h
index c9e226cef..ede35a561 100644
--- a/Marlin/Configuration_adv.h
+++ b/Marlin/Configuration_adv.h
@@ -2790,6 +2790,8 @@
   //#define GCODE_QUOTED_STRINGS  // Support for quoted string parameters
 #endif
 
+//#define GCODE_CASE_INSENSITIVE  // Accept G-code sent to the firmware in lowercase
+
 /**
  * CNC G-code options
  * Support CNC-style G-code dialects used by laser cutters, drawing machine cams, etc.
diff --git a/Marlin/src/gcode/parser.cpp b/Marlin/src/gcode/parser.cpp
index 5a473403a..0fac65300 100644
--- a/Marlin/src/gcode/parser.cpp
+++ b/Marlin/src/gcode/parser.cpp
@@ -118,11 +118,18 @@ void GCodeParser::parse(char *p) {
 
   reset(); // No codes to report
 
+  auto uppercase = [](char c) {
+    #if ENABLED(GCODE_CASE_INSENSITIVE)
+      if (WITHIN(c, 'a', 'z')) c += 'A' - 'a';
+    #endif
+    return c;
+  };
+
   // Skip spaces
   while (*p == ' ') ++p;
 
   // Skip N[-0-9] if included in the command line
-  if (*p == 'N' && NUMERIC_SIGNED(p[1])) {
+  if (uppercase(*p) == 'N' && NUMERIC_SIGNED(p[1])) {
     #if ENABLED(FASTER_GCODE_PARSER)
       //set('N', p + 1);     // (optional) Set the 'N' parameter value
     #endif
@@ -135,7 +142,7 @@ void GCodeParser::parse(char *p) {
   command_ptr = p;
 
   // Get the command letter, which must be G, M, or T
-  const char letter = *p++;
+  const char letter = uppercase(*p++);
 
   // Nullify asterisk and trailing whitespace
   char *starpos = strchr(p, '*');
@@ -271,7 +278,7 @@ void GCodeParser::parse(char *p) {
     bool quoted_string_arg = false;
   #endif
   string_arg = nullptr;
-  while (const char param = *p++) {              // Get the next parameter. A NUL ends the loop
+  while (const char param = uppercase(*p++)) {  // Get the next parameter. A NUL ends the loop
 
     // Special handling for M32 [P] !/path/to/file.g#
     // The path must be the last parameter
@@ -289,14 +296,14 @@ void GCodeParser::parse(char *p) {
       }
     #endif
 
-    // Arguments MUST be uppercase for fast GCode parsing
     #if ENABLED(FASTER_GCODE_PARSER)
-      #define PARAM_TEST WITHIN(param, 'A', 'Z')
+      // Arguments MUST be uppercase for fast GCode parsing
+      #define PARAM_OK(P) WITHIN((P), 'A', 'Z')
     #else
-      #define PARAM_TEST true
+      #define PARAM_OK(P) true
     #endif
 
-    if (PARAM_TEST) {
+    if (PARAM_OK(param)) {
 
       while (*p == ' ') p++;                    // Skip spaces between parameters & values
 
diff --git a/Marlin/src/gcode/parser.h b/Marlin/src/gcode/parser.h
index 70eb85e31..7c5e1518b 100644
--- a/Marlin/src/gcode/parser.h
+++ b/Marlin/src/gcode/parser.h
@@ -166,7 +166,6 @@ public:
     #ifdef CPU_32_BIT
       FORCE_INLINE static bool seen(const char * const str) { return !!(codebits & letter_bits(str)); }
     #else
-      // At least one of a list of code letters was seen
       FORCE_INLINE static bool seen(const char * const str) {
         const uint32_t letrbits = letter_bits(str);
         const uint8_t * const cb = (uint8_t*)&codebits;
@@ -177,14 +176,27 @@ public:
 
     static inline bool seen_any() { return !!codebits; }
 
-    #define SEEN_TEST(L) TEST32(codebits, LETTER_BIT(L))
+    FORCE_INLINE static bool seen_test(const char c) { return TEST32(codebits, LETTER_BIT(c)); }
 
   #else // !FASTER_GCODE_PARSER
 
+    #if ENABLED(GCODE_CASE_INSENSITIVE)
+      FORCE_INLINE static char* strgchr(char *p, char g) {
+        auto uppercase = [](char c) {
+          return c + (WITHIN(c, 'a', 'z') ? 'A' - 'a' : 0);
+        };
+        const char d = uppercase(g);
+        for (char cc; (cc = uppercase(*p)); p++) if (cc == d) return p;
+        return nullptr;
+      }
+    #else
+      #define strgchr strchr
+    #endif
+
     // Code is found in the string. If not found, value_ptr is unchanged.
     // This allows "if (seen('A')||seen('B'))" to use the last-found value.
     static inline bool seen(const char c) {
-      char *p = strchr(command_args, c);
+      char *p = strgchr(command_args, c);
       const bool b = !!p;
       if (b) value_ptr = valid_float(&p[1]) ? &p[1] : nullptr;
       return b;
@@ -192,12 +204,12 @@ public:
 
     static inline bool seen_any() { return *command_args == '\0'; }
 
-    #define SEEN_TEST(L) !!strchr(command_args, L)
+    FORCE_INLINE static bool seen_test(const char c) { return (bool)strgchr(command_args, c); }
 
     // At least one of a list of code letters was seen
     static inline bool seen(const char * const str) {
       for (uint8_t i = 0; const char c = str[i]; i++)
-        if (SEEN_TEST(c)) return true;
+        if (seen_test(c)) return true;
       return false;
     }
 
@@ -205,7 +217,7 @@ public:
 
   // Seen any axis parameter
   static inline bool seen_axis() {
-    return SEEN_TEST('X') || SEEN_TEST('Y') || SEEN_TEST('Z') || SEEN_TEST('E');
+    return seen_test('X') || seen_test('Y') || seen_test('Z') || seen_test('E');
   }
 
   #if ENABLED(GCODE_QUOTED_STRINGS)