Aimar came off the bench for the last 15 minutes of River's 2-0 win over Rosario Central in the Argentine Primera Division, having left the club for Spain in 2000.
The 35-year-old was given a rousing reception as he replaced Leonardo Pisculichi, with coach Marcelo Gallardo thrilled to see his former team-mate wearing River's famous red-and-white shirt again.
"The feeling in the air was the joy of seeing a child of the house returned after a long time with great effort," Gallardo said.
"He had the opportunity to be happy on the pitch, which was what we all wanted. We are happy for his appearances. Hopefully this is the beginning, to follow and to have continuity."
Aimar played five seasons with River Plate after making his debut in 1996, winning five league titles.
After a brief spell in Malaysia, he returned to the club in January but repeated injury problems delayed his debut.