Transit 3B was an American satellite which was launched in 1961 and operated by the United States Navy.

The launch resulted in a partial failure, with the spacecraft being placed in a more eccentric orbit than planned, and failing to separate from the upper stage.

[3] Transit 3B was a 113-kilogram (249 lb) spacecraft, which was powered by 6,600 solar cells charging a nickel-cadmium battery.

Due to the low perigee of its orbit, Transit 3B decayed quickly, and reentered the atmosphere on 30 March 1961, less than 37 days after launch.

[3] The systems demonstrated by Transit 3B included a digital clock to control timing signals, uploading programs into the onboard computer's memory whilst in orbit, data retention and FM encoding, and the use of phase modulation for data transmission without affecting the satellite's Doppler signal.