The 21-year-old Kieboom, who was rated the 98th-best prospect in baseball by ESPN's Keith Law prior to this season, drilled a solo shot to dead center off Padres reliever Craig Stammen to lead off the bottom of the eighth inning. The first hit of his career, Kieboom's bomb tied the game at 3-3 and resulted in a standing ovation from the crowd of 27,000-plus.
"I'm not sure I've never even dreamed that, but that was unbelievable," said Kieboom, who went 1-for-4 on the night. "That's like the cherry on top of anybody's call-up moment. That was real special. I really appreciate the fans for doing that."
In the bottom of the ninth, with Washington trailing 4-3 after San Diego's Hunter Renfroe homered off Nats closer Sean Doolittle, Kieboom came up with two outs and the bases loaded. Facing Padres closer Kirby Yates, he worked the count to 2-2 before going down swinging. Despite the game-ending whiff, Kieboom's teammates were still impressed with the young infielder.
"That's a heck of a ballgame for his debut," said Nats ace Max Scherzer, who fanned 10 in seven innings, including his 2,500th career strikeout. "To go out there and his first hit being a solo shot, and tying the ballgame up in a big spot -- he did great tonight. You know he's on cloud nine. In your debut game, everybody is. For him to not only to be in a debut game, to stay calm and collected in a big spot and hit a homer, that's awesome. Something he'll never forget."
Kieboom was off to a torrid start at Triple-A Fresno, batting .379 with three home runs and 18 RBIs in 18 games, with a 1.142 OPS.
"This whole thing is crazy to me," Kieboom said prior to his debut, in which he batted ninth and played shortstop. "I was in high school three years ago, doing art projects and stuff.
"It's a moment that I think every kid dreams about, and it happened real quick. Next thing you know, I'm here. So this is real special."
Friday's contest also marked the return of third baseman Anthony Rendon, who missed four games after taking a pitch off his left elbow. Rendon was hit by another pitch in his first plate appearance against San Diego, and went 0-for-3. To make room on the roster, the Nationals sent Jake Noll back to Fresno on Thursday and moved right-handed pitcher Koda Glover to the 60-day injured list.
In addition, the Nationals placed reliever Trevor Rosenthal on the 10-day injured list with what they're calling a viral infection. He has struggled this season, giving up 12 runs on seven hits and nine walks in just three innings.
Historically, the Nationals have brought top prospects to the big leagues only when there's an opportunity to play every day -- as was the case when they called up Juan Soto from Double-A last year.
"Like we usually do with our top prospects, he's going to play a lot," Nationals general manager Mike Rizzo said Friday. "We're going to be excited to see him perform. Hopefully, he helps us win games."
With the addition of Kieboom to the lineup, the Nationals started three position players age 21 or younger (Victor Robles, Soto, Kieboom). The last team to do that was the 1997 Marlins.
"I think that's all any player has ever wanted when they get called up, is an opportunity to show what they can do," Kieboom said. "But to know that I'll be given a legit opportunity to play is a huge factor. It gives you confidence. ... I don't know what the whole game plan is exactly, but I know I'm starting tonight and that's all and that's where I am at first."
Despite a strong showing in spring training, including a game with two home runs off Houston Astros ace Justin Verlander, Kieboom didn't make the major league roster as the Nationals wanted him to continue to work on his defense. A natural shortstop, the 28th overall pick of the 2016 draft committed 26 errors in 118 games last year.
With Trea Turner entrenched as Washington's starting shortstop, Kieboom logged time at second base during spring training and at Fresno. In six games playing second for the Grizzlies, he made one error. He also had one miscue in 12 games at shortstop.
Due to injuries in the infield, the Nationals decided to promote Kieboom. Just four games into the season, Turner fractured his right index finger after being hit by a pitch while attempting to bunt on April 2. On Friday, Rizzo said he is "weeks away" from returning as opposed to "months."
The expectation is that Kieboom will take over as the primary shortstop until Turner returns from injury, allowing Wilmer Difo to fill in around the infield as needed. Veteran second baseman Brian Dozier has gotten off to a slow start offensively. Kieboom's older brother, Spencer Kieboom, is a catcher in the Nationals organization and appeared in 52 games last season for Washington. He is currently at Double-A Harrisburg.