The 84-year-old leader had quit the Congress after nearly 50 years in January after expressing his unhappiness for being sidelined.
"I am humbled today that I am joining the party which was led by great leaders. I see great India emerging under the leadership of Prime Minister Narendra Modi and ably supported by the party President Amit Shah," Krishna told reporters after joining the saffron party at its headquarters here.
He said he always enjoyed cordial relationships with BJP leaders since the time when Atal Bihari Vajpayee was the prime minister.
Welcoming Krishna into the BJP, Shah said his joining is a message to all honest politicians that they should strengthen the hands of Prime Minister Modi.
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Terming the Krishna's decision to join the BJP as "timely one", Shah said, "Krishna is joining the party after witnessing change in the country since Modi became Prime Minister. Since then the party's acceptability across the country has gone up."
Krishna's induction is also seen as a big boost to the BJP, which is now looking to spread its footprint in southern India ahead of the assembly elections in Karnataka next year.
The party will also give him due respect keeping in mind his seniority, the BJP president added.
While quitting the Congress, Krishna, former External Affairs Minister, said the party is in confusion whether it needed mass leaders or not, in clear indication of him being upset with the party.
Krishna was Karnataka chief minister between 1999 and 2004. He has also served as the Maharashtra governor. He had returned to state politics after stepping down as External Affairs Minister in 2012.