China's Baidu Inc said on Wednesday it has agreed to buy the app store of Hong Kong-listed Netdragon Websoft Inc for $1.85 billion in cash in what would be the biggest deal in China's IT sector.

Baidu's purchase of 91 Wireless, one of the country's top three app stores by monthly users, fits in with the diversification strategy of China's biggest internet services provider.

Baidu said it expects to close the deal by the fourth-quarter of this year. "The acquisition of 91 Wireless significantly strengthens our mobile app distribution capability," said Chief Executive Robin Li.

Baidu, which has a market capitalization of $49.5 billion, said it had purchased a 57.4 percent of 91 Wireless from Netdragon and the rest from other shareholders.

Baidu shares have risen 41 percent since the beginning of the year, outperforming the NASDAQ 100 Index which is up 18 percent in the same period.