European Union leaders agreed Thursday to make Ukraine an official candidate to join the bloc, opening the door to possible membership in the years to come.

The decision was agreed to by EU leaders at a summit in Brussels and fulfills one of Ukrainian President Volodymyr Zelensky’s biggest requests of European countries. It also is intended to send a resounding message of support for Kyiv to the Kremlin.

EU leaders also made Moldova—which borders Ukraine and where Russian troops are present in its secessionist Transnistria region—an official candidate to join the bloc.

German Chancellor Olaf Scholz, center, said the EU needs institutional reforms before it can take on new members.

Photo: Olivier Hoslet/Shutterstock

After EU leaders made the decision on Ukraine, Mr. Zelensky was connected into the room to join the meeting.

Mr. Zelensky told the leaders that their move was one of the most important decisions for Ukraine in the 30 years of its independence.

“However,” he said, “I believe this decision is not only for Ukraine. This is the biggest step towards strengthening Europe that could be taken right now…when the Russian war is testing our ability to preserve freedom and unity.”

The decision on Ukraine and Moldova came after the European Commission, the bloc’s executive body, last week recommended offering both countries so-called candidate status, an unthinkable prospect before Russia’s invasion of Ukraine on Feb. 24.

Backing from Germany, France and Italy for the move, announced when the leaders of those countries visited Kyiv last week, ultimately proved critical in sweeping away hesitation about a membership pathway for Ukraine and Moldova in the Netherlands, Portugal, Denmark and other Western European countries.

Nonetheless, the offer is only the first step of a long and uncertain accession bid for Ukraine and Moldova that could take decades and has no guarantee of success. The commission set a series of overhaul tasks for both countries before any accession negotiations can begin.

The last country to join the bloc was Croatia in 2013, and there is little likelihood of the EU’s acceptance of new members over the next few years.

Even the most optimistic of EU diplomats doubt any actual membership talks will start before 2024. German Chancellor Olaf Scholz and French and Italian leaders have said the EU would need significant changes—and possibly a revision in its treaty rules—before it could absorb several new members. Previous efforts to change treaty rules have dragged on for years, derailed by no votes in referendums in various countries.

“We must make the European Union able to embrace new members,” Mr. Scholz said Wednesday in Germany’s Parliament. “That requires institutional reforms. And we should use those reforms to promote democracy and to strengthen the rule of law in the European Union as well.”

The EU’s vote of confidence in Ukraine comes at a difficult moment in the war against Russia, with Ukraine in danger of ceding critical ground in the country’s east. Mr. Zelensky has said an EU path would provide a boost for Ukrainians’ morale in the war, as well as opening the possibility of becoming entrenched in Western institutions when the conflict is over. The opening of the country’s accession prospects will deliver no immediate additional European help.

The EU leaders approved the commission’s list of conditions for Ukraine to fulfill before the EU can start membership talks with Kyiv. The list includes appointments to top courts and the country’s anticorruption body, securing minority rights and enforcing an anti-oligarch law.

The commission set similar goals to fight corruption, improve judicial independence and crack down on organized crime for Moldova, which Russian officials have suggested could be a target for military action.

Ukraine, Moldova and Georgia lodged EU membership bids in the first days of Russia’s invasion of Ukraine. In early March, the bloc’s leaders discussed it at a summit in Versailles, France, with only a few speaking in favor. However, Mr. Zelensky’s continued appeals for a membership path have gradually shifted opinion.

The EU decided not to approve candidate status for Georgia because of resistance from the government to agreed domestic reforms.

“It’s an important moment for the European continent,” said Belgian Prime Minister Alexander de Croo. Giving Ukraine candidate status “is a very significant symbolic message but at the same time, it doesn’t mean that Ukraine will soon be an EU member. It’s a process of many years.”

Once accession talks are open, Ukraine and Moldova will have to complete a plethora of painstaking reforms, from strengthening the rule of law to tackling defunct state-run companies. It took Croatia five years to complete the accession talks and another two before it became a member.

Even as the EU opened the pathway to Ukraine and Moldova, the bloc demonstrated the continued troubles it is facing in putting its long-held enlargement plans on track. Support for enlargement has soured in many Western European countries over concern about an influx of cheap labor and worries that the bloc is becoming too big and unwieldy.

At a meeting Friday morning with leaders of several western Balkan countries, which have been seeking membership of the bloc for the better part of two decades, EU leaders again failed to agree to open accession negotiations with North Macedonia and Albania. Bulgaria has been blocking the talks because of historic differences with North Macedonia. The former Yugoslav republic was first granted EU candidate status in 2005.

EU officials have acknowledged that failure to advance membership talks with Balkan countries has put the bloc’s credibility on the line and undermined the ability of the accession process to produce economic and political reforms in candidate countries.

“Today, we should be launching negotiations with Albania and North Macedonia, and I cannot hide my disappointment, certainly,” said EU foreign-policy chief Josep Borrell.

After the meeting, Albanian Prime Minister Edi Rama posted on Twitter a family-photo picture of the EU and Balkan leaders together.

“Nice place nice people nice words nice pictures and just imagine how much nicer could be if nice promises were followed by nice delivery,” he wrote above the picture.

Write to Laurence Norman at laurence.norman@wsj.com