Yes. God is truly just because He gives to all existing things what is proper to the condition of each (and preserves the nature of each in the order and with the powers that properly belong to it).
Since good as perceived by intellect is the object of the will, it is impossible for God to will anything but what His wisdom approves. This is, as it were, His law of justice, in accordance with which His will is right and just.
Hence, what He does according to His will He does justly: as we do justly what we do according to law. But whereas law comes to us from some higher power, God is a law unto Himself.