Michael Fallon, the Defense Secretary, said the new agreement was a signal Britain would "stand firm" with the beleaguered eastern European nation, The Telegraph wrote.
The 15-year-long agreement will see British troops take part in more joint exercises and carry out more training with Ukrainian forces who have been battling Russian-backed separatists for two years.
The countries will also cooperate on sharing military intelligence and expertise.
Read alsoBritain to help Ukraine counter Russian propagandaMr. Fallon said: "The UK will stand firm with Ukraine as they defend their territorial integrity. This new defense agreement sets out that commitment as we enhance our training of Ukrainian armed forces."
The new agreement revives an earlier pact that lapsed in 2006 because of the anti-EU stance of the then pro-Russian Ukrainian president, Viktor Yanukovych, officials said.