The Prime Minister revealed his thoughts on the ambitious proposal as he spoke to schoolchildren playing with a model container ship onboard lighthouse tender NLV Pharos on the Thames.
Mr Johnson told the children that he had recently been discussing the possibility of constructing a bridge over the Irish Sea.
He said: "(I was talking yesterday) about building a bridge from Stranraer in Scotland to Larne in Northern Ireland - that would be very good.
"It would only cost about £15 billion."
It follows reports that the Prime Minister has asked government officials for advice on the costs and risks of such a project.
The DUP - who prop up the Prime Minister's minority government - support proposals for a bridge which they hope could bring economic benefits to Northern Ireland and Scotland, but it has also been touted as a potential solution to the controversial backstop.
Mr Johnson first mooted the idea while serving as foreign secretary, telling the Sunday Times last year: "What we need to do is build a bridge between our islands. Why don't we? Why don't we?
"There is so much more we can do, and what grieves me about the current approach to Brexit is that we are just in danger of not believing in ourselves, not believing in Britain."
Mr Johnson last year reportedly suggested also building a bridge across the English Channel to connect the UK and France.