The teenage forward opened his account for the club in Pamplona just five minutes after coming on as a substitute. He got across his marker to head home and make it 1-1, with the game eventually ending 2-2.
Aged 16 years and 304 days, his goal sees him take the crown from Bojan Krkic, whose first league goal for Barca came when he was 17 years and 53 days old. Lionel Messi's arrived at 17 years and 331 days old.
However, Ansu is not La Liga's youngest-ever scorer. Fabrice Olinga, playing for Malaga at the time, scored the winning goal against Celta Vigo in 2012 when he was 16 years and 98 days old. Next comes Athletic Bilbao's Iker Muniain, who was 16 years and 289 days old when his first league goal arrived, with Ansu now placing third on the list.
Ansu had become the youngest player to represent Barcelona in the league since the 1940s when he came on in last week's win over Real Betis.
He began preseason with the club's U19 team and coach Victor Valdes revealed he didn't even have a decent pair of boots when he turned up for a training camp in Russia.
"Ansu arrived at the camp with pain in his feet so I asked him to show me his boots," Valdes explained earlier this week. "They were a genuine disaster. The pain was coming from there. We went and bought him new ones.
"But I can't speak highly enough of him: he's strong, quick, daring... we have to give him complete freedom so that all flows. He's got character, too, and he's lethal around the box."
However, despite the youngster's record goal, Barca boss Ernesto Valverde was left fuming with the result.
"We did not play well at all, we were awful," Valverde told a news conference after his side were left with only four points from their opening three games.
"They went ahead and we couldn't get close to their area. Then we dominated the play in second half, we took the lead and had the game in our hands but they came back at us and levelled with that absurd penalty.
"We cannot afford to let a game like this one slip, we should have been more assertive on the pitch and kept on attacking."
Barca defender Pique said he felt hard done by to be penalised late in the game, when the ball bounced off his arm as he tried to block a cross.
"For me that's not a penalty, I had no time to remove my arm, I was not taking up more space than usual, I jumped in a natural way," he said.
His goal at El Sadar cancelled out Roberto Torres' opener, with Arthur Melo then firing Barca into the lead. However, Osasuna grabbed a point late on when Torres added his second from the penalty spot after Gerard Pique's handball.
Pique admitted Barca had made a much worse start to the season than they expected but said there was no reason to panic.
"We would have liked to have taken nine points at this stage and we're having more problems than we expected to but we're not worried," he added.
"It's going to be a very long season and the important thing is that when we get to March or April we have a chance of winning the league."
Barca have now dropped points in two of their opening three league games this season, following their loss to Athletic in the opening round of fixtures two weeks ago.