Speaking ahead of the fourth anniversary of Russia's full-scale invasion of Ukraine, Johnson told the BBC that the troops should be sent to peaceful regions in non-fighting roles.
The comments from Johnson, who was one of Ukraine's staunchest supporters as Britain's leader during the first months of the conflict, were contained in excerpts of an interview that will be broadcast on Sunday.
If adopted, the proposal from Johnson would mark a major policy shift for the UK and its allies.
While the "coalition of the willing" is working on plans to send troops to Ukraine, such a deployment would only take place after an agreement to end the fighting and would be designed to police the ceasefire.
"If we are willing to do it in the context of a ceasefire, which of course puts all the initiative, all the power, in Putin's hands, why not do it now?'' Johnson said.
"There is no logical reason that I can see why we shouldn't send peaceful ground forces there to show our support, our constitutional support for a free, independent Ukraine."
Western military planners have avoided discussing such a move publicly out of concern that Russia would see it as an escalation of the conflict.
Putin, in September, rejected western proposals for a peacekeeping force in Ukraine, saying that any troops deployed to the country would be "legitimate targets".
But Johnson said Putin shouldn't be able to dictate terms to Ukraine and its allies.
"It's about whether Ukraine is a free country or not," he said.
"If it's a vassal state of Russia, which is what Putin wants, then obviously it's up to Putin to decide who comes to his country. If it's not, then it's up to the Ukrainians."
When asked about Johnson's comments, Britain's Defence Ministry said the government was continuing to work with the coalition of the willing to prepare for the deployment of troops to Ukraine after a ceasefire.
"The multinational force Ukraine under UK leadership will secure peace for the long term, with the prime minister being clear that we will put British troops on the ground following the end of hostilities," the ministry said in a statement.
Johnson said the war in Ukraine was the result of the west's failure to support Ukraine after the invasion of Crimea in 2014, as well as its inability to punish Bashar al-Assad for using chemical weapons against his own people in Syria and the debacle of the US withdrawal from Afghanistan.
"I think Putin was emboldened by a western failure in Syria to punish Assad for using chemical weapons," he said.
"I think Putin was further emboldened in February 2022 by what he'd seen in Afghanistan, and a sort of general sense that the west was on the back foot. He'd seen those appalling pictures of Americans being forced to flee Afghanistan and the UK pulling out as well, and that really did embolden him."
