A weak pound, fears of recession in the UK and surging sales at French luxury goods makers are thought to be behind the shift, according to data from Bloomberg.
It’s the first time Paris has overtaken London since records began in 2003.
It comes as the UK is expected to fall into recession this year, although the French economy is also under pressure.
The combined value of British shares is now around $2.821 trillion (£2.3 trillion), while France’s are worth around $2.823 trillion, Bloomberg calculates.
It marks a huge reversal of fortunes for the London Stock Exchange, which was worth about $1.4 trillion more than its Parisian rival back in 2016.
France has been catching up for some time but shares in the UK’s medium sized companies have been doing particularly badly this year, as consumers cut back their spending and businesses struggle with higher costs.