Burberry and The Home Depot are the only two retail brands testing the service, while other partners include Eminem, Pharrell and activist organisation Global Citizen.
The new button will only show to a "small percentage" of Twitter's US users for now, with further brand partnerships to follow.
Twitter's group product manager, Tarun Jain, said the button will give brands the chance to turn the relationship with existing followers into sales. A key difference for brands using the "buy now" button rather than a promoted tweet is that products will only appear to existing followers, who may be more likely to convert.
One possible roadblock may be user trust. Twitter confirmed to Marketing that this is the first time it will be storing user payment details in its own systems. Though the company has experimented with e-commerce before through partnerships with Starbucks and American Express, payment details were previously stored with partners.
Now Twitter has updated its security and privacy policies to reassure users that their payment and shipping details will be held securely and encrypted, meaning they'll only have to enter their payment information once. Twitter will also ask for permission before sharing card details with retailers. Whether users entrust their details to an unproven retail player may act as a bellwether for the future of social commerce. Facebook introduced a similar "buy now" feature in July.
Twitter has partnered with e-commerce companies Fancy, Stripe, Gumroad and Musictoday to build out the underlying platform, with more partners to follow.