Tamari (Best Option)
1 tablespoon per 1 tablespoon soy sauceTamari is a Japanese soy sauce variant that is typically gluten-free and has a richer, less salty profile than traditional soy sauce. It contains similar amino acids and umami compounds, which contribute to the savory depth in meatballs. This makes it an excellent direct substitute.
When using tamari, monitor salt levels as it can be slightly less salty; you may want to adjust added salt accordingly. It also integrates well with the meat mixture without affecting texture.
The final meatballs will have a very similar flavor and color, maintaining the characteristic savory and slightly sweet notes expected from soy sauce, making it the closest match.