Bedi parted ways with Kejriwal after he decided to float a political party in October 2012 and was severely critical of his decision.
"I have been fond of Kiran Bedi ji. I always tried to convince her that she should join politics. I am happy she did it today," Kejriwal tweeted.
However, at AAP's press briefing, party leader Sanjay Singh, termed her joining BJP as "shocking".
Kejriwal had even invited her to join AAP and be the party's chief ministerial candidate before assembly polls in 2013 which she had declined.
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Bedi today said she joined BJP due to the inspirational leadership of Prime Minister Narendra Modi.
Asked whether BJP may project her as the chief ministerial candidate, Singh said "It does not look so."
65-year-old Bedi was inducted into the BJP fold in the presence of party President Amit Shah and Union ministers Arun Jaitley and Harsh Vardhan at the party's headquarters.
Bedi was highly critical of Kejriwal, especially when he staged a dharna outside the Rail Bhavan during his 49-day government.
She had been hailing Modi and supporting BJP through her Twitter posts since the start of campaign for Lok Sabha polls last year.