Broadcast Ownership for the 21st Century Act - Directs the Federal Communications Commission (FCC) to modify current cross-ownership limitations by eliminating provisions limiting the granting or renewal of an AM or FM radio or television broadcast license to any party on the basis of the ownership, operation, or control of a daily newspaper. Bars the FCC from prohibiting or limiting a person or entity from holding any form of ownership or other interest in a broadcasting station and a cable system serving the same community.

Amends the Communications Act of 1934 to provide that, in calculating the national audience reach limitations, UHF stations shall be attributed with no more than 50 percent of the television households in their market.

Amends the Telecommunications Act of 1996 to direct the FCC to modify its rules for multiple ownership of TV broadcast stations to increase to 45 (currently 35) percent the national audience reach limitations for TV stations owned by the same entity or person.

Directs the FCC to revise its rules for local television multiple ownership to allow a person to own, operate, or control two TV stations in the same market area if the grade B contours of such station: (1) do not overlap; or (2) do overlap and at least six independent broadcast or cable television voices would remain in the area after transfer of the license of the stations in question.