Obama and South Korean President Lee Myung-bak both called on North Korea to cancel the rocket test.
The two leaders spoke at a joint press conference Sunday ahead of a two-day nuclear summit.
The planned launch was announced after a recent deal that would ship U.S. food aid to the North in exchange for a moratorium on missile and nuclear tests.
Obama said: "Bad behavior will not be rewarded."
Obama and Lee said that North Korea's past violations of U.N. resolutions have tightened sanctions against the Pyongyang government. Obama said he suspects that will happen again.
