The 60-year-old former city worker was charged with killing 10 people from 1974 to 1991 and eluding capture for 30 years. The plea came as Rader's trial was set to get under way in Wichita.
Wearing body armor beneath a suit, Rader testified in a flat voice he killed to fulfill sexual fantasies, and described struggling with frantic victims, strangling them and photographing their bodies.
He said some victims were chosen at random, while others were people he chose and watched.
Some victims' family members wept as he spoke in the hushed room, the Wichita Eagle reported.
The self-nicknamed "BTK" killer sent a series of letters and rambling poems to the media and police beginning in 1974 and ending in February with Rader's arrest.