The British military presence in Estonia will be bolstered as NATO dramatically escalates its ability to respond to Vladimir Putin's Russia.
Leaders from the 30-member alliance are meeting in Madrid for a summit where they will agree to co-operate more closely and significantly increase the number of troops held at high readiness.
The UK already has a significant military presence in Estonia and the Prime Minister will use the summit to expand its headquarters in the Baltic nation.
Officials said this would ensure the UK could provide rapid reinforcements if needed, and deploy artillery, air defence and helicopters.
The alliance plans to have 300,000 troops at high readiness – up from the current 40,000 – and the UK will commit capabilities in land, air and sea to the "new force model".
Mr Johnson told reporters: "We've already got in Estonia a very significant enhanced forward presence of two battlegroups.
"We're working with premier Kaja Kallas on what we can do to be more supportive to Estonia, to help them operationally.
Watch: Russia is the most direct and pressing threat to Europe, says Defence Secretary.
"The work is going on for a close political and military partnership. Our commitment to Estonia, like our commitment to all our NATO friends, is absolute."
Mr Johnson will also use the Madrid summit to push for greater defence spending across the alliance, despite domestic rows over the UK's own military funding.
NATO members have a commitment to spend at least 2% of gross domestic product (GDP) – a measure of the size of the economy – on defence, but only nine of the 30-member alliance meet that requirement.
Mr Johnson claims the UK will spend 2.3% this year and will push for allies to do more.
He said: "The NATO alliance keeps our people safe every day. But over the next 10 years the threats around us are only going to grow.
"We need allies – all allies – to dig deep to restore deterrence and ensure defence in the decade ahead.
"The 2% was always meant to be a floor, not a ceiling, and allies must continue to step up in this time of crisis."
Mr Johnson faces ministers and military chiefs in open revolt about the level of UK spending.
The new head of the Army, General Sir Patrick Sanders, said any further cuts to the size of the British Army – which is set to shrink from a target figure of 82,000 troops to 72,500 – would be "perverse".
Watch: Exercise Hedgehog: UK troops help Estonia prepare Russia defence.
Defence Secretary Ben Wallace said: "For too long defence has lived on a diet of smoke and mirrors, hollowed-out formations and fantasy savings when in the last few years threats from states have started to increase.
"It is now time to signal that the peace dividend is over and investment needs to continue to grow before it becomes too late to address the resurgent threat and the lessons learned in Ukraine," he added.
"It is time to mobilise, be ready and be relevant."
There was a boost for NATO as the summit got underway, with membership for Sweden and Finland moving a step closer.
The UK had supported the Swedish and Finnish bids to join the alliance.
Mr Johnson said they were "breaking decades of historic neutrality” to join the organisation, which showed the alliance was "in robust health".
"Sweden and Finland's membership will make our brilliant alliance stronger and safer," he said.
Watch: Former Deputy Secretary General of NATO Rose Gottemoeller believes NATO are well prepared for expansion.