Small businesses will soon be able to sell shares to Main Street investors on crowdfunding portals, instead of trying to lure those backers with promises of T-shirts, coffee mugs or other merchandise.

Starting Monday, small companies and startups can raise as much as $1 million online from ordinary investors in a 12-month period. Until now, federal securities laws allowed only wealthy individuals, or so-called accredited investors, to participate in such offerings. The new fundraising option stems from the 2012 Jumpstart Our...