Apinis threw 20.83 meters, pulling out his best result of the season just when it was required.
This is the fifth Paralympics for Apinis, who has won four golds, two silvers and two bronze medals in the discus and shot-put disciplines over the course of his career.
His performance saw congratulations pouring in from officials and was particularly welcome given Latvia's failure to win a single medal in the earlier Rio Olympic Games.
The well-wishers were led by President Raimonds Vejonis who described it as a "beautiful victory".
Latvija patiesi lepojas ar Aigara Apiņa zelta panākumu #Rio2016Paralympics! Skaista uzvara!— Valsts prezidents (@Rigas_pils) September 8, 2016