On 30 November 1945 "Sęp" was again officially commissioned into the Polish Navy. In 1946 she was rearmed with Soviet caliber torpedoes and guns. In 1951 eight crew members were accused of plotting to defect with the ship to Sweden, and were prosecuted in a Stalinist show trial. In a 1959 Polish film the ship was used to portray her twin . In 1959 the submarine became a training ship. She remained the largest submarine of the postwar Polish Navy until 1962 when it commissioned the first of four Soviet-built s, which were of similar size. In 1964 she suffered a serious fire (8 crewmembers died), after which she was repaired, but was not fully operational. In 1969 the ship suffered another accident while submerged. The ship was decommissioned on 15 September 1969 and subsequently scrapped in 1972.