The editor-turned-director, who is married to Shah Rukh's close friend and choreographer-director-and-now-actor Farah Khan, had created a stir last year when he took a potshot at the superstar's mega-budget venture "RA.One" on Twitter.
"I just heard a 150 Cr firework fizzle," Kunder had tweeted.
However, Kunder has now apologised for the same in a tweet today, and says he regrets it.
"I'll always regret one thing that I feel very guilty about. I sincerely apologise for my tweet on 'RA.One'," he tweeted.
In January this year, Kunder and Shah Rukh were caught in a brawl at actor Sanjay Dutt's bash. However, the two called truce.
Recently, Shah Rukh also got photographs clicked with the aliens of Kunder's new directorial venture, "Joker". This act sparked speculation that the actor is helping in promoting the film.
However, Farah, who was present at a post-release event for her film, "Shirin Farhad Ki Toh Nikal Padi", in which she makes her debut as an actor, lost her cool when asked if Shah Rukh was promoting her husband's film.
"Where have you seen Shah Rukh Khan promoting the film? Has he come on any channel to promote it? He just took one picture with the aliens. Please don't spread nonsense," she said.
"Joker", which is due to release on August 31, stars Sonakshi Sinha and Akshay Kumar.
Even though Farah was not aware of the tweet, she said she was proud of her husband because she felt that it took a lot of courage to apologise.
"I haven't read the tweet right now. But I am proud of my husband. It takes a big heart to apologise," she said.