When the Treaty of Tartu was signed on 14 October 1920, Finland agreed to return the Russian icebreakers that the Finnish White Guard had seized during the Civil War in 1918.

Since both the size of the ships calling at the Finnish winter ports and the amount of exported goods, especially forest products, had increased considerably since the First World War, there was a definite need for a large and powerful icebreaker.

[3] In 1923, the state allocated FIM 10 million for the design and development of a new icebreaker based on the experiences gained during the four years that Wäinämöinen had spent under the Finnish flag.

The basic design of the new vessel, which was to have a beam of at least 18 metres (59 ft), was awarded to experienced Finnish naval architects K. Albin Johansson and Ossian Tybeck.

The most expensive offer was received from the Finnish Sandvikens Skeppsdocka och Mekaniska Verkstads Ab that was already building another new icebreaker, Voima, in Helsinki.

Although there were still issues with the oil-fired boilers by late February, it was agreed to take the delivery of the new icebreaker on 2 March 1926 and solve the problems later as the winter of 1926 had turned out to be very severe.

After Sampo had arrived from Hanko with the Minister of Commerce and Industry, Tyko Reinikka, other representatives of the state, and members of the press, the newest and largest state-owned icebreaker demonstrated her power by easily overcoming the notorious pack ice fields surrounding the island and escorting a convoy of three merchant ships to the open seas.

Captain Juho Lehtonen, who had replaced Rosqvist in July 1926, even received hate mail from Swedish-speaking citizens, some of whom questioned his professional competence after a minor collision with a Swedish steamer Yrsa on 18 January 1927.

Although she was often attacked by Soviet bombers, the enemy never scored a direct hit on the icebreaker, giving her a reputation of a lucky ship among the Finnish seafarers.

After the Winter War ended with the 1940 Moscow Peace Treaty, Jääkarhu opened a channel to the icebound port of Hanko and helped to evacuate the city and the surrounding islands that had been leased to the Soviet Union as a naval base.

After returning from a dry dock in Stockholm, Sweden, Jääkarhu resumed her duties in the Gulf of Finland, and together with Sisu she escorted almost 700 merchant ships through the ice-infested waters.

[14] She was also used in the 1969 Soviet/Italian film The Red Tent to represent another Soviet icebreaker, Krasin, which had participated in the rescue mission of Umberto Nobile and other survivors of the crash of airship Italia in 1928.

[15] Her bow, which had slightly rounder lines than the previous Finnish icebreakers,[16] had a stem angle of 23–25 degrees and her hull, protected by an ice belt up to 28.6 millimetres (1.13 in) thick, was divided into eight compartments by watertight transverse bulkheads.

Unlike the previous coal-burning icebreakers, Jääkarhu had eight oil-fired boilers with mechanical ventilation that consumed 2.5 to 4.5 tons of fuel oil per hour.

Although she was the most expensive to operate, and for that reason she was always the last one to enter service and first one to sail back to her summer moorings, her endurance and range were considerably better than those of the older icebreakers since she could hold nearly 1,000 tons of fuel and required refueling only two or three times during normal winters.

In addition, she had six centrifugal pumps for moving 100 tons of water in five minutes between the fore and aft peak tanks to adjust her trim and release the icebreaker from compressive ice.