"It's an important part of our strategy and something we are actively working on today not only alone but talking to other partners who'd also like to see more choice for how they can monetise on the phone," Spencer said.
Spencer was having a chat during an interview in Sao Paulo during the CCXP comics and entertainment convention.
He didn’t say when the store would kick-off, but it could be next year.
"I don't think this is multiple years away, I think this is sooner than that,'' Spencer said.
Microsoft's mobile store would enter a challenging regulatory climate around smartphone-based digital marketplaces.
Fortnite-maker Epic Games has sued Apple and Alphabet's Google over their iOS and Android store practices, alleging they are unnecessarily restrictive and unfair. Apple doesn't allow competing stores on its iPhone and iPad platforms and collects a 30 per cent cut of sales for most purchases.
Game makers have taken issue with the fees.
Microsoft might be able to use the anger that developers and users have to bolster support for its store offering.
Xbox's cloud gaming technology lets users stream blockbuster games to mobile phones. "We've talked about choice, and today on your mobile phones, you don't have choice,'' Spencer said.
"To make sure that Xbox is relevant today but for the next 10, 20 years, we're going to have to be strong across many screens."