The Labour leader said he would "invest properly" in the armed forces, promising more ships for the Royal Navy and aircraft for the RAF.
"We will protect the people of this country from any threat that they face anywhere in the world," he said.
In Friday's Question Time special, Mr Corbyn faced hostile audience questions about his stance on nuclear weapons.
Labour has committed to replacing the Trident missile system in its manifesto, despite Mr Corbyn's anti-nuclear views, and the leader has been reluctant to say whether he would be prepared to authorise a strike if Britain was under attack, if he was PM.
During a visit to Lincoln he was asked if he would keep the British people safe as prime minister.
He said he would, and added: "We will invest properly in our police service, we will invest properly in our armed services, the numbers in the armed services have gone down, the navy are crying out for more ships, the air force are crying out for more surveillance aircraft.
"We would fund them properly to achieve all of that."
He also repeated his vow to pursue a foreign policy aimed at addressing the issue of "ungoverned spaces" in war-torn countries like Libya where extremism can flourish.
Speaking after the Question Time show, Conservative Foreign Secretary Boris Johnson said Mr Corbyn's approach to Trident meant Labour's backing for the armed forces was "completely meaningless because when it came to the business of defending this country he wouldn't do it".
Jeremy Corbyn has vowed to keep the UK safe after facing questions about whether he would use nuclear weapons.