diff --git a/src/core/symbology/qgsmarkersymbollayer.cpp b/src/core/symbology/qgsmarkersymbollayer.cpp
index fea1803009bc..feb442bb7e67 100644
--- a/src/core/symbology/qgsmarkersymbollayer.cpp
+++ b/src/core/symbology/qgsmarkersymbollayer.cpp
@@ -39,6 +39,8 @@
 Q_GUI_EXPORT extern int qt_defaultDpiX();
 Q_GUI_EXPORT extern int qt_defaultDpiY();
 
+static constexpr int MAX_FONT_CHARACTER_SIZE_IN_PIXELS = 500;
+
 static void _fixQPictureDPI( QPainter *p )
 {
   // QPicture makes an assumption that we drawing to it with system DPI.
@@ -3174,8 +3176,20 @@ void QgsFontMarkerSymbolLayer::startRender( QgsSymbolRenderContext &context )
     mFont.setStyleName( QgsFontUtils::translateNamedStyle( mFontStyle ) );
   }
 
-  const double sizePixels = context.renderContext().convertToPainterUnits( mSize, mSizeUnit, mSizeMapUnitScale );
+  double sizePixels = context.renderContext().convertToPainterUnits( mSize, mSizeUnit, mSizeMapUnitScale );
   mNonZeroFontSize = !qgsDoubleNear( sizePixels, 0.0 );
+
+  if ( mNonZeroFontSize && sizePixels > MAX_FONT_CHARACTER_SIZE_IN_PIXELS )
+  {
+    // if font is too large (e.g using map units and map is very zoomed in), then we limit
+    // the font size and instead scale up the painter.
+    // this avoids issues with massive font sizes (eg https://github.com/qgis/QGIS/issues/42270)
+    mFontSizeScale = sizePixels / MAX_FONT_CHARACTER_SIZE_IN_PIXELS;
+    sizePixels = MAX_FONT_CHARACTER_SIZE_IN_PIXELS;
+  }
+  else
+    mFontSizeScale = 1.0;
+
   // if a non zero, but small pixel size results, round up to 2 pixels so that a "dot" is at least visible
   // (if we set a <=1 pixel size here Qt will reset the font to a default size, leading to much too large symbols)
   mFont.setPixelSize( std::max( 2, static_cast< int >( std::round( sizePixels ) ) ) );
@@ -3405,6 +3419,9 @@ void QgsFontMarkerSymbolLayer::renderPoint( QPointF point, QgsSymbolRenderContex
     transform.scale( s, s );
   }
 
+  if ( !qgsDoubleNear( mFontSizeScale, 1.0 ) )
+    transform.scale( mFontSizeScale, mFontSizeScale );
+
   if ( mUseCachedPath )
   {
     p->drawPath( transform.map( mCachedPath ) );
@@ -3514,6 +3531,7 @@ QRectF QgsFontMarkerSymbolLayer::bounds( QPointF point, QgsSymbolRenderContext &
   {
     chrWidth *= scaledSize / mOrigSize;
   }
+  chrWidth *= mFontSizeScale;
 
   bool hasDataDefinedRotation = false;
   QPointF offset;
diff --git a/src/core/symbology/qgsmarkersymbollayer.h b/src/core/symbology/qgsmarkersymbollayer.h
index 2f9adb28d92a..d092776c6830 100644
--- a/src/core/symbology/qgsmarkersymbollayer.h
+++ b/src/core/symbology/qgsmarkersymbollayer.h
@@ -996,6 +996,8 @@ class CORE_EXPORT QgsFontMarkerSymbolLayer : public QgsMarkerSymbolLayer
 
     double mChrWidth = 0;
     QPointF mChrOffset;
+    //! Scaling for font sizes, used if font size grows too large
+    double mFontSizeScale = 1.0;
     double mOrigSize;
 
     QColor mStrokeColor;
diff --git a/tests/src/core/testqgsfontmarker.cpp b/tests/src/core/testqgsfontmarker.cpp
index a865c111860b..e56bb1a494bb 100644
--- a/tests/src/core/testqgsfontmarker.cpp
+++ b/tests/src/core/testqgsfontmarker.cpp
@@ -60,6 +60,7 @@ class TestQgsFontMarkerSymbol : public QObject
     void bounds();
     void fontMarkerSymbolDataDefinedProperties();
     void opacityWithDataDefinedColor();
+    void massiveFont();
 
   private:
     bool mTestHasError =  false ;
@@ -219,6 +220,26 @@ void TestQgsFontMarkerSymbol::opacityWithDataDefinedColor()
   QVERIFY( result );
 }
 
+void TestQgsFontMarkerSymbol::massiveFont()
+{
+  // test rendering a massive font
+  mFontMarkerLayer->setColor( QColor( 0, 0, 0, 100 ) );
+  mFontMarkerLayer->setStrokeColor( QColor( 0, 0, 0, 0 ) );
+  QFont font = QgsFontUtils::getStandardTestFont( QStringLiteral( "Bold" ) );
+  mFontMarkerLayer->setFontFamily( font.family() );
+  mFontMarkerLayer->setDataDefinedProperties( QgsPropertyCollection() );
+  mFontMarkerLayer->setCharacter( QChar( 'X' ) );
+  mFontMarkerLayer->setSize( 200 );
+  mFontMarkerLayer->setSizeUnit( QgsUnitTypes::RenderMillimeters );
+  mFontMarkerLayer->setDataDefinedProperty( QgsSymbolLayer::PropertySize, QgsProperty::fromExpression( QStringLiteral( "if(importance > 2, 100, 350)" ) ) );
+  mFontMarkerLayer->setDataDefinedProperty( QgsSymbolLayer::PropertyLayerEnabled, QgsProperty::fromExpression( QStringLiteral( "$id in (1, 4)" ) ) ); // 3
+  mFontMarkerLayer->setStrokeWidth( 0.5 );
+
+  bool result = imageCheck( QStringLiteral( "fontmarker_largesize" ) );
+  mFontMarkerLayer->setDataDefinedProperties( QgsPropertyCollection() );
+  QVERIFY( result );
+}
+
 //
 // Private helper functions not called directly by CTest
 //
diff --git a/tests/testdata/control_images/symbol_fontmarker/expected_fontmarker_largesize/expected_fontmarker_largesize.png b/tests/testdata/control_images/symbol_fontmarker/expected_fontmarker_largesize/expected_fontmarker_largesize.png
new file mode 100644
index 000000000000..4b7955744af3
Binary files /dev/null and b/tests/testdata/control_images/symbol_fontmarker/expected_fontmarker_largesize/expected_fontmarker_largesize.png differ