The declaration came at the conclusion of Wednesday EU-India summit in Brussels.
It comes days after terrorist attacks on Brussels Zaventem airport and Maelbeek metro station, which killed 32 people and injured scores of others.
The leaders "strongly condemned" the attacks on 22 March 2016 as an "unacceptable affront to our open democratic societies."
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A joint statement issued after the one-day summit, the 13th between the two sides, read, "The EU and India confirmed their commitment to remain united and firm in the fight against hatred, violent extremism and terrorism."
Despite EU-India relations having been strained recently, due to a long running legal dispute concerning two Italian marines who shot dead in 2012, two Indian fishermen who they mistook for pirates.
The leaders were committed to stepping up EU-India cooperation to counter extremism and radicalisation, stem the flow of foreign terrorist fighters and curb sources of terrorist financing and arms supply.
The summit, the first for four years, involved: European Commission President Jean-Claude Juncker, European Council President, Donald Tusk, and Indian Primes Minister Narendra Modi.
It was Modi’s first visit to Brussels and other issues on the agenda included migration, climate change, clean energy, a water partnership, terrorism and the stalled EU-India free trade and investment agreement. The summit was also an opportunity for the leaders to discuss human rights issues.
The case of the Italian marines, which has blocked closer EU-India relations, is now under international arbitration under the UN Convention on the Law of the Sea (UNCLOS). There are also similar cases relating to 14 Estonians and six Britons who have been arrested in relation to anti-piracy operations. On Wednesday, Italy launched a legal battle to repatriate the marines.
During the summit, it was announced by European Investment Bank (EIB) president Werner Hoyer, a massive loan to India of €450 million to "strengthen engagement to support long-term investment in India."
The EU is India’s largest trading partner, accounting for 13% of India’s overall trade (in 2015 the total value of EU-India trade in goods reached €77.5 billion) and primary foreign investor.
The economic links between the two sides were highlighted by Indian giant Tata Steel's announcement on the eve of the summit that it plans to sell its British assets, with thousands of jobs at stake.
The leaders from the two sides said they were "re-engaged" in discussions on "how to further" the EU-India Broad-based Trade and Investment Agreement (BTIA) negotiations.
The EU and India launched negotiations on a free-trade agreement in 2007 but progress has been held up due to differences over intellectual property rights and efforts by Brussels to link trade with climate change action.
Reaction to this came from UK ECR MEP and chairman of the European parliament's delegation for relations with India MEP Geoffrey Van Orden, who met Modi before the summit started, said "Completion of the free trade agreement with India has taken far too long. Negotiations have been on and off for nine years now and I hope the visit of Modi will at last accelerate the process. This is the top priority."
On the joint declaration on counter terrorism, Van Orden said, "Both India and many of our European countries have suffered from terrorism and have much expertise in dealing with such challenges."