Kosmos 521 (Russian: Космос 521 meaning Cosmos 521), also known as DS-P1-M No.4 is a satellite which was intended for use as a target for tests of anti-satellite weapons.

[1] No attempt to intercept the satellite was made after its onboard telemetry system malfunctioned.

[2][5] Western analysts did not identify Kosmos 521 as being associated with the Soviet ASAT programme until records were declassified.

Kosmos 521 was the last of the five original DS-P1-M satellites to be launched,[1] of which all but the first were successfully reached orbit.

Despite its successful launch, the satellite malfunctioned shortly after reaching orbit, and was unusable.