The 44-year-old director has named his daughter Roohi and his son Yash, after his late father Yash Johar.
Taking to Twitter, the stars congratulated Johar on the arrival of his little ones.
Alia, who considers Johar her mentor and guide, was the first one to wish the filmmaker on social media.
"Finally I can say I have a younger brother AND sister!!! So so so happy!!! So much love to give uff bursting with joy," she tweeted.
Farah posted, "Glad you took my advice seriously @karanjohar Best thing to happen to you... And they'll be the youngest people you hang out with so all's good."
Sidharth Malhotra, who made his Bollywood debut with Johar's "Student of the Year" alongside Alia and Varun Dhawan, is elated with the decision of his first time director.
"So happy you took this decision Johar! It's a full house now. You will be an amazing father, love to your new big family," he tweeted.
Riteish Deshmukh said, "Heartiest Congratulations my dearest @karanjohar thrilled beyond words. Love to little angels Roohi & Yash & big big hug to Hiroo aunty."
Sushant Singh Rajput, who is busy shooting Johar's next film "Drive", congratulated the producer on his biggest day.
"I'd such a big smile this morning when I read about this. Lots and lots of love @karanjohar and a big hearty congratulations," Sushant wrote.
Arjun Rampal tweeted, "@karanjohar Congratulations Johar, this is the best news in years. You will make an amazing parent. Love to the little ones. God bless."
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