* Offer unchanged at 3,660 yen per share
* Tender offer deadline extended to Dec. 6
* ADK shares jump over 5 pct
* WPP could reinvest in ADK once offer completed (Adds detail on new agreement)
TOKYO, Nov 21 (Reuters) - Bain Capital LLC on Tuesday said its $1.35 billion offer for Asatsu-DK Inc (ADK) had been accepted by the Japanese company's 25 percent shareholder WPP, after the parties floated the idea of WPP reinvesting at a later date.
Led by the high-profile businessman Martin Sorrell, WPP clashed with the U.S. private equity firm Bain last month when it sought to buy Japan's third-largest advertising agency, which partners with WPP. WPP said the offer undervalued the firm.
Bain said on Tuesday WPP had now agreed to sell its ADK shares for 3,660 yen ($32.53) each, the same price it rejected last month.
WPP and Bain would now discuss the possibility of WPP holding about 25 percent of an entity that would own ADK, the Japanese agency said in a statement on Tuesday.
ADK shares jumped more than 5 percent to 3,660 yen in Tokyo on Tuesday after initial reports of the deal, as investors bet on a successful offer by Bain for at least 50.1 percent.
The U.S. private equity fund said it had extended the offer to Dec. 6.
Bain's initial offer came after Japan's ADK asked it to buy WPP's stake to end a two-decade business alliance which ADK said failed to produce synergy.
WPP sought a higher price and filed for arbitration, seeking a declaration that ADK's plan to terminate the alliance was invalid and that ADK had no right to request or require WPP to sell its shares.
WPP would end arbitration and injunction proceedings if the tender offer succeeds, Bain said in its statement.
Other ADK shareholders such as London-based fund manager Silchester International Investors LLP and Hong Kong-based activist hedge fund Oasis Management Co Ltd have also previously said Bain's offer was too low.
Oasis declined to comment. Silchester could not be reached for comment. ($1 = 112.5100 yen)
(Reporting by Junko Fujita in TOKYO and Kate Holton in LONDON; Editing by Christopher Cushing)