The "Saved By the Bell" alum and "Franklin & Bash" star shared the news on Twitter on Wednesday morning.
"So in love with our new baby girl, Lachlyn!" he tweeted. "Born last night 2/17 Mum and baby are healthy and happy. 2 boys, 2 girls, too perfect!"
"She's a 'mouth-breather' like her Daddy. My sweet little angel," Gosselaar later posted on Instagram, sharing a sweet snapshot of a sleeping Lachlyn.
That makes two kids for Gosselaar and his second wife Catriona McGinn. The couple first welcomed son Dekker Edward Gosselaar in October 2013.
The former teen heartthrob wed his ad exec wife in July 2012 at a Santa Ynez winery.
The actor's other two kids -- son Michael, 11, and daughter Ava, 8 -- are from his 14-year marriage to Lisa Anna Russell, which ended in May 2011.
Gosselaar took son Michael backstage with him during his Bayside High reunion with Thiessen, Mario Lopez and
"Here's @jimmyfallon with the 11 year old me tonight. Wait...what?!" he wrote.