The decision was made at a party congress in Astana.
Addressing delegates, QAP leader Azat Peruashev justified the move by saying "the interests of the nation" should take precedence over everything else.
President Nazarbaev, who attended the congress, said he expected more parties to merge with Otan in the future.
Kazakhstan has now 10 officially registered parties, most of which back the president.
Nazarbaev today said the country should have fewer but stronger parties that could "efficiently defend the interests of the population."
Asar, (All Together), the political grouping of Nazarbaev's daughter Darigha, merged with Otan in July.
Otan's membership is expected to reach nearly 1 million with today's merger.
Kazakhstan has a population of 15.3 million.
(Kazakhstan Today, Interfax-Kazakhstan)
READY TO LEAD THE OSCE? Kazakhstan has been lobbying to be granted the rotating chair of the Organization for Security and Cooperation in Europe (OSCE) in 2009. In March, current OSCE head Karel De Gucht visited the country and said Kazakhstan is better prepared to head the OSCE than any other Central Asian country. But concerns about the state of democracy, the media, and ethnic relations make Kazakhstan's bid extremely controversial....(more)