At the outset of the First World War, the new naval base in Tallinn was incomplete, which is why the Russians began to station vessels of their Baltic Fleet in Viapori. Work to construct fortifications was also initiated in Helsinki, meaning that the plan dating back to the Swedish era for fortifying the mainland was now being realised. While Viapori was not subjected to fighting during the First World War, the overthrow of the Emperor in Russia in late winter 1917 caused unrest in the fortress.

After the First World War broke out, the Russians had time to construct coastal fortifications and lay minefields in the Gulf of Finland. The threat of landing in Viapori was regarded as unlikely. The Russians thought that the Germans might land troops somewhere on the western coast of Finland, advancing via Finland in order to launch an assault on St. Petersburg. The Russians built field fortifications in an area that extended from the southern coast up to Nurmes in North Karelia.

The function of the fortress of Viapori changed. Not only the Viapori naval port but also the City of Helsinki needed to remain in the hands of the defender. The Russians needed to be able launch a flank manoeuvre from the fortress against the Germans while they were advancing towards St. Petersburg and to stymie any opportunity for collaboration between the German troops and the German navy. While the land front was expanded, the Russians also began to build increasingly durable fortifications. The new defense line was built ten kilometres away from the naval port. As the war progressed and the Germans did not attack, a third line of fortifications was built north and east of Helsinki.

During the war, the number of seamen aboard ships in the Viapori naval port amounted to 25,000 to 30,000. Furthermore, around 15,000 soldiers were stationed in Viapori and Helsinki and the surrounding area.

In accordance with the Brest-Litovsk peace treaty, signed in March 1918, the Russian Baltic Fleet left Helsinki in four contingents by May 1918, and the Viapori naval port at Katajanokka was transferred to the state of Finland.