No catcher has hit that many out of the park. Only 40 previous sluggers had reached the 400 plateau.
The ball was presented to Piazza by Robert Hodge of Vista, Calif., after the game, and the catcher reciprocated with a signed bat.
"In New York," he said, grinning, "they drive a harder bargain."
The last time he'd signed a bat, Piazza mentioned, was for a gentleman who'd won the Medal of Honor for the Battle of the Bulge.
Piazza had the ball in his locker. After jokingly suggesting he'd sell it on eBay, the slugger said it would be preserved in a special place.
Asked to describe what it meant, Piazza, a 62nd-round pick by the Dodgers in the 1988 First-Year Player Draft as a favor to Tommy Lasorda, paused in thought.
"You do want to get it out of the way -- you don't want it to be an albatross," Piazza said, the homer his third as a Padre in 57 at-bats. "It's good to get it done and turn the page. I'm not looking towards 500 yet ... but I think I have a few more in me. I've been fooling them this long.
"I'm more philosophical the last few years. Being a late-round pick, my whole career has been a blessing. I'm more reflective, pragmatic. I've caught a lot of games, had a lot of injuries -- even today, getting conked.
"It's a very humbling feeling as well. I don't want to get too deep or sentimental, but if you work hard and stay focused, you can live your dream. There were 1,300 guys drafted ahead of me. I look at it that way when I talk to kids, [to] motivate them. You can live a dream.
"Stay positive. I've always maintained a good attitude. You're going to have frustration. But to me, you have to embrace frustration to make the good times better."
Piazza was "conked," as he put it, by Shawn Green's bat in his backswing during the fourth inning. The catcher went down and stayed down, waiting for clarity to return to his brain.
"I don't know how he clipped me, but he rang my bell," Piazza said. "I was a little loopy. I felt nauseous for three innings. I had to get smelling salts. They asked me if I wanted to come out, but felt it was best to stick it out. If I'm a boxer, I would have been down on points.
"It's frustrating when that happens. It's a straight blow to the head. Every now and then you get hit like that. [Gary] Sheffield, around '99, got me in the head and cut it open."
Somehow, the incident made 400 all the more meaningful. The man has caught 1,546 games, sixth all-time in the National League.
"Ultimately, you strive for consistency -- not just myself, but the team," Piazza said, clearly not happy with his .228 average or six RBIs in 15 games. "You need a streak -- 9 out of 12 or 13 -- to get that positive momentum going. It's just tough to get that momentum.
"Hopefully, we'll play better as a team and get on a roll."
From Piazza's viewpoint, there's no time like the present.
The Dodgers are coming to town for three games.