The personal finance journalist sells his website for £87m, pledging £10m of the proceeds to charity. But not all of the site's users are convinced by the deal.
MoneySavingExpert.com, which was set up by Mr Lewis in 2003 and now sends a weekly email to around 5 million subscribers, is to be bought by price comparison website MoneySupermarket.
The site, which employs 42 people, was now "far bigger than the man who founded it" and it was the right time to sell, Lewis said.
Mr Lewis, who is well known as a television pundit on money matters, will receive £60m upfront in a mixture of cash and shares, and a further £27m conditional on meeting targets over the next three years.
He plans to donate £10m to charity from the deal, including £1m to Citizens Advice, while he will retain full control over the website.
In a message on his website, he said: "This is great news for MoneySavingExpert.com and its users, ensuring, with or without me, the site will be around for many years to come, maintaining our ethos of 'cutting your costs and fighting your corner'."
Mr Lewis has waged a number of high-profile campaigns on behalf of consumers, including refunding mis-sold payment protection insurance (PPI).
Reaching a wider audience
Peter Plumb, chief executive officer of MoneySupermarket, said the website shared a common goal of helping customers save money.
"We'll help MoneySavingExpert.com reach a wider audience and MoneySavingExpert.com will broaden the range of advice and tools we offer, encouraging even more people to take action, tap into the benefits of the internet to find a better deal and make the most of their money."
Some site users were not convinced by the deal, however. On the site's forums, sablade described it as a sellout, adding: "To start with we will see no difference but drip by drip moneysupermarket will suffocate this site and strip it of all its ways to turn the public's free advice into pure profit."
Fen_boy asked: "Are the regular and extremely helpful forum users getting a dividend then? That's where most of the value lies in this site. You're all being sold to line Martin's pockets otherwise."