Martha Stewart is stepping into "The Real Housewives of New York City" reboot ... TMZ has learned.

Sources connected to the series tell TMZ ... Martha recently filmed a cameo for season 16, appearing alongside her longtime makeup artist, Daisy Toye. We're told the shoot went down within the last couple of weeks, and the cameo is already in the can.

Daisy, one of the newest additions to the cast, has worked closely with Martha for years ... making the crossover a natural fit.

Toye and Hailey Glassman -- best known as Jon Gosselin's ex -- joined the rebooted series. Returning cast members include Erin Lichy, Sai De Silva and Jessel Taank, with newcomer Erika Hammond and returning alum Carole Radziwill also in the mix.

As for Hailey's Bravo debut, we reported Gosselin isn't planning to tune in ... but it's not about their past. A source close to Jon told us he's just not into 'Housewives,' noting he's more of a sports guy ... so there’s no interest there.