The 21-year-old Kieboom, who was rated the 98th best prospect in baseball by ESPN's Keith Law prior to this season, 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, who will bat in the No. 9 spot vs. the Padres, said Friday. "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."
The Nationals also got good news with the return of third baseman Anthony Rendon, who missed four games after taking a pitch off his left elbow.
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 only brought top prospects to the big leagues 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.
"He's got a high baseball IQ. He really knows how to play the game," Rizzo said. "There's going to be a learning curve. There's going to be hiccups. We think we have a special 21-year-old player that we think is going to allow us to compete and assists us in winning some games."
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.