Schauffele carded four birdies and four bogeys at The Renaissance Club while winning his second consecutive event. He won the Travelers Championship at the end of June.
The 7-under 273 total resulted in the seventh career PGA Tour victory for the 28-year-old Schauffele, who also won the Olympic gold medal in Japan last year.
"It was stressful. I'm not going to put makeup on a pig here," said Schauffele, who also won an unofficial pro-am event in Ireland earlier in the week.
"It was a very average day, probably my worst of the week."
Schauffele shot 65 in the second round and 66 in the third to get in position for victory.
He made a crucial par save on the par-3 17th hole to take a two-shot lead to the 18th. The cushion was needed, as he missed a 7-foot par putt. He then tapped in for bogey to clinch his third win of the season.
Only Scottie Scheffler has won more wins this season with four. Schauffele is tied with Sam Burns for second.
The San Diego State product was thrilled to win on the famed course.
"It's special. It's different playing over here," Schauffele said.
"You've got to play golf differently. The fans are incredible. They were pushing me along all day. This is definitely a nice win for my team and myself."
The toughest part of the day for Schauffele was when he bogeyed three holes during a four-hole stretch to end the front nine. But instead of letting his round disintegrate, he responded with six pars and two birdies over the next eight holes to all but secure the tournament.
"I was not playing very well," Schauffele said.
"I came out of the gates very nicely and kind of hit a wall and was looking a bit sketchy for a little bit, as I like to say, and it was nice to sort of steady the ship on the way in."
Kitayama posted his second 66 of the week - the other came in the first round - to keep the heat on Schauffele. He was the clubhouse leader as Schauffele completed his round.
"I just knew that I had to get off to a good start," Kitayama explained.
"Yesterday I kind of fumbled a little bit. So I knew I could do it in these kind of conditions if I was able to be steady all the way through."
South Korea's Joohyung Kim shot 67 to finish alone in third at 5 under. Patrick Cantlay and England's Tommy Fleetwood also shot 67s and finished in a tie for fourth at 4-under 276.
Kim had four birdies and his lone bogey occurred on the last hole.
"I played really solid today," Kim said. "Other than that, I played bogey-free until the last hole and 18 is one of the toughest holes we have this week and it was that last putt, just broke a little bit, obviously with the nerves and stuff."
Cantlay started brightly with four birdies over the first seven holes. But he had just one more while shooting two bogeys to end up three shots back.
Still, it was his best round of the tournament.
"I thought I played really well pretty much the whole time," Cantlay said.
"I didn't get the most out of every round, and that was probably the difference but other than that, you know, all in all, really solid week."
Fleetwood had four birdies and his lone bogey also came on 18.
"Overall today, played great, so I have to be happy with that," Fleetwood said.
"I think I improved every day and throughout the week. Whatever happens, it's a really good result and it's a positive week."
Kitayama, Brandon Wu and Wales' Jamie Donaldson qualified for The Open Championship as the top three finishers not already part of next week's field.
Wu (67 on Sunday) and Donaldson (67) tied for sixth at 3-under 277 with Cameron Tringale (70) and England's Matt Fitzpatrick (70).
Jordan Spieth was among a six-way tie for 10th at 2 under. He shot a final-round 72.
The Open begins on Thursday at the iconic St Andrews as the tournament celebrates its 150th event. The prize purse stands at $14,000,000 for the 2022 edition.
