Kyiv has faced heavy losses on the battlefield after two years of war, whereas Russia has benefited from a sizeable advantage in manpower.
The move will allow Ukraine to call up more people to replenish its reserves, after volunteer numbers dropped.
Mr Zelensky said in December that 500,000 more soldiers were needed.
Although the bill was voted through by MPs in May 2023, it did not come into force because Mr Zelensky did not sign it.
It was not immediately clear what prompted the president to sign the bill on Tuesday, however he has previously warned of plans Russia may have to launch a spring or summer offensive this year.
Mr Zelensky’s signing of the bill could signify Ukraine’s effort to build up strong defensive fortifications in preparation for a Russian offensive.
Ukrainian forces had hoped to take back swathes of territory gained by Russia as well as cutting off Russian supply lines to Crimea.
But lacking air superiority and faced with formidable Russian defences, Kyiv’s counter-offensive ground to a halt at the start of winter and there are fears that Russian forces could outgun Ukraine.
Last year, Ukrainian Gen Oleksandr Tarnavskyi warned troops had already been forced to downsize some military operations because of a drop-off in foreign aid.
Ukraine has relied heavily on Western supplies, particularly on deliveries of long-range missiles and air defence systems. However, billions of pounds of US and EU aid have been held up amid political wrangles.
Russian President Vladimir Putin has said more than 600,000 of his troops are fighting in Ukraine, but has not revealed how many have lost their lives there.
A classified US intelligence report released in December estimated that 315,000 Russian soldiers had been either killed or wounded since the war began – which it said was almost 90% of Russia’s military personnel at the start of the invasion.
In February, Mr Zelensky said 31,000 Ukrainian soldiers had been killed since the start of the war, but US officials have put the number of Ukrainian soldiers killed as at least 70,000 and as many as 120,000 injured.