The first ice of the winter was seen in the second week of November in the innermost bays of the northern Bay of Bothnia. November had been warmer than usual until then, and some temperature records had been broken. Consequently, the first ice melted, and the sea was free of ice for some time. The actual increase in the amount of ice did not start until the end of November and early December. Ice appeared in the Gulf of Finland slightly before mid-December when new ice was observed in Vyborg Bay and outside St. Petersburg.

Mild temperatures prevailed in December, as they had in November. With southerly winds and heavy cloudiness prevailing, the sea-ice growth was moderate. The first icebreaker to start assisting shipping was Kontio, which began operations after Christmas. At this time the ice in the Bay of Bothnia, off Oulu, Kemi, and Tornio, was 5-10 cm thick, and assistance restrictions were issued for the harbours. A small amount of ice had also formed in the Vaasa Archipelago and in the internal archipelago of the southern part of the Bay of Bothnia. Otherwise, Finnish waters were ice-free, and the sea water was 2-3 degrees warmer than average.

In January, the prevailing wind was northerly to south-easterly and temperatures were wintry as a consequence. The coldest period in January was in the middle of the month and ice formed quickly along the entire Finnish coast. On 23 January the icebreaker Voima started its operations in the Gulf of Finland as assistance restrictions also took effect in the harbours of the Gulf of Finland and the Sea of Bothnia. In the previous winter no icebreaking services were needed in the Gulf of Finland. In the Bay of Bothnia the ice was up to 40 cm thick at this point. After the most severe freezing weather that lasted for less than a week, the cold eased somewhat and temperatures were mild, especially in southern sea areas. The amount of ice increased steadily in the Bay of Bothnia, but in other sea areas the amount of ice hardly grew at all at the end of the month. There were four stormy days in January, which is a regular amount.

In February cold air flowed from the north and the wind was occasionally strong. This caused the ice in the Bay of Bothnia to drift southward. Despite the freezing temperatures, a large ice-free area appeared in the Bay of Bothnia, through which shipping was possible over a long distance without icebreaker assistance. Factors leading to the emergence of a large area of open water probably included the very warm temperatures in 2020, which led to high enough heat content in the water column to prevent freezing. However, the freezing temperatures continued, and the ice cover expanded. In the second week of the month ice had formed up to the centerline of the Gulf of Finland, and there was also new ice in the outer parts of the Archipelago Sea. The Quark was covered by drift ice already at the beginning of the month. The Saimaa Canal was closed on 8 February. The largest ice extent occurred on 15 February, covering an area of 127,000 km². At that time the Bay of Bothnia was completely covered by ice, and there was also ice along the coast of Germany and in the Straits of Denmark. At the end of February weather that had been dominated by high pressure gave way to low-pressure areas from the North Atlantic, and warmer air flowed in from the southwest. In the Åland Islands the air temperature rose to ten degrees, and temperatures also briefly went above the freezing point in the Bay of Bothnia. There was also heavy snowfall in February. A thick snow cover slowed the thickening of the ice.

The very warm days that prevailed at the end of February were reflected in weak ice in places along the southern coast. At the same time westerly winds had pushed the ice field in the Bay of Bothnia against the Finnish coast and the drift ice in the Gulf of Finland stayed mostly in Russian waters. Soon after the beginning of March temperatures dropped again, and the winter's second more extensive phase of ice formation occurred.

On 10 March the ice cover was 84,000 km². However, higher temperatures toward the end of the month meant that March ended up being warmer than usual and the ice cover started to rapidly shrink. At the end of the month there was little to no drift ice in the offshore areas of the Quark, the Sea of Bothnia, and the Gulf of Finland, and the ice in the Archipelago Sea had started to rot.

In early April temperatures remained high, and by the 12th of the month, the Sea of Bothnia and the Archipelago Sea were free of ice. The last assistance restrictions were also lifted in the Gulf of Finland at that time. The ice field in the northern part of the Bay of Bothnia remained firmly in place along shipping lanes leading to Finnish harbours until the wind turned to north on 23 April. The northerly winds were strong for five days. The ice drifted to sea areas off Kokkola and Kalajoki, which had lost its ice earlier. After this, the wider ice fields never returned to the north, and the ice gradually melted in place. The Gulf of Finland was ice-free before the end of the month. The last remaining ice in the Bay of Bothnia melted on 20 May.

The first freezing occurred along the coasts later than usual. In the northern part of the Bay of Bothnia, on the southern edge of the Archipelago Sea, and in the eastern part of the Gulf of Finland, the first ice formed nearly at the normal time, but in other sea areas one to four weeks later than usual.

The final disappearance of the ice occurred earlier than usual. In the Bay of Bothnia and the Sea of Bothnia the ice disappeared one to two weeks earlier than usual, except for the sea off Kokkola, where the ice remained longer than usual. In the Gulf of Finland, the ice disappeared a week and a half to three weeks earlier than usual.

In the Gulf of Bothnia there were 19–43 fewer ice days than usual, and in the Gulf of Finland, between 9 and 17 days fewer.

The greatest thickness for fast ice varied in the Bay of Bothnia between 40 and 70 cm, in the Sea of Bothnia from 20 to 45 cm, and in the Gulf of Finland from 20 to 50 cm. In the open sea the thickness of the ice in the Bay of Bothnia was between 10 and 50 cm, in the Sea of Bothnia it was 5 to 20 cm, and in the Gulf of Finland 15 to 35 cm.

The distance that vessels had to navigate through ice on 15 February 2021 was 194 nautical miles from Kemi, 205 nautical miles from St. Petersburg to the ice edge, and from Riga, the distance was 37 nautical miles. Correspondingly the navigation distance in ice 15 cm thick or more was 95 nautical miles, 16 nautical miles and from Riga, the entire distance was in ice less than 15 cm thick.