* Monex to pay $9.75 per share, or about $411 million
* Price at 32 pct premium to TradeStation share price on April 20
* Tender offer for shares to begin by May 10 (Adds background)
TOKYO, April 21 (Reuters) - Japanese online brokerage Monex Group said it will buy U.S. peer TradeStation Group for about $411 million to gain a foothold in the world's largest equities market.
Monex, Japan's No.2 online broker by the number of accounts, will offer $9.75 per share, which is a 32 percent premium to TradeStation's closing stock price on April 20. It will start a tender offer for all outstanding shares on May 10.
The move overseas comes as Monex and other Japanese online brokers struggle amid bleak growth prospects due to the sluggish economy and shrinking population, as well as tough price competition.
Shares of Monex slumped 2.1 percent to 16,700 yen, reflecting investor worries over the financial burden.
"We have enough cash for this acquisition and we see no impact on our financial health from this," Monex Chief Executive Officer Oki Matsumoto said at a briefing.
TradeStation is one of the six publicly traded online brokers in the U.S. and caters mainly to individual investors, with 48,000 accounts, Monex said in a statement.
It is far smaller than the biggest players in the market such as Charles Schwab Corp , which boasts more than 10 million accounts and TD Ameritrade Holdings Corp with 8 million customers.
Monex, which is more than 20 percent owned by Japanese leasing firm Orix Corp , has also taken steps to bolster its presence in China, including its purchase of Hong Kong-based broker Boom Securities.
Major stakeholders in TradeStation include units of institutional investors BlackRock Inc and Fidelity.
J.P. Morgan is acting as financial adviser for TradeStation on the deal, while Deutsche Bank is acting as financial adviser for Monex.
(Reporting by James Topham, Junko Fujita and Emi Emoto; Editing by Chris Gallagher and Nathan Layne)