TNCC president B S Gnanadesikan said AICC General Secretary and party's in-charge for Tamil Nadu Mukul Wasnik had informed him of the decision over phone.
"Indian National Congress has decided to support Kanimozhi...Accordingly, all five Congress MLAs will vote for her," he said in a statement.
The term of six Rajya Sabha MPs from Tamil Nadu ends next month.
The eight-year allies had parted ways in March this year over the Sri Lankan Tamils issue, and today's development, being speculated for the past few days, is seen as part of a political realignment ahead of next year's scheduled Lok Sabha polls.
DMK has 23 MLAs in the 234-member Tamil Nadu assembly and seven candidates, including CPI's D Raja, are in the fray for the biennial elections. AIADMK has fielded four candidates and DMDK one.
AIADMK with 150 members strength is sure to win four seats. Each winning candidate has to muster 34 first preference votes for victory.