Target is taking its marketing talents mainstream. After quietly creating its own "guest access" platform, which offers targeted marketing opportunities for Target vendors last year, the retailer is now opening the product up to all national advertisers. The Minneapolis-based company will host a launch party Thursday afternoon in Manhattan to announce the program's expansion to advertisers beyond those found on Target shelves, including automotive brands, financial firms and travel companies.

"What we have packaged is access to our guests," said Kristi Argyilan, who joined Target two years ago as senior VP-media and guest engagement. "We use first-party data for a very active shopper who has a ring of influence that is important—there are a lot of marketers that would love the opportunity to be able to place ads in front of those consumers," she said. Competitors like Amazon and Walmart also operate similar offerings for advertisers.

The 1,795-unit Target has about 80 employees devoted to the platform and also works with a few external tech partners. Thus far, the chain has collaborated with eight "alpha" vendor partners, including pharmaceutical company GlaxoSmithKline, and a host of other brands on different levels, Ms. Argyilan said.

In one recent spring campaign for GSK's Flonase allergy product, Target was able to isolate likely purchasers based on last year's shopping data. It the marketed to them with targeted messaging via digital channels like banner ads and its Cartwheel loyalty app, as well as media sources from Target's own Bullseye media marketplace. The two-month-long campaign resulted in a 40% sales lift for those shoppers exposed to the campaign compared with a control group who did not receive any ads. Among allergy product purchasers with kids at home, the campaign had a 50% sales lift, Ms. Argyilan noted.

"We built this in house for us and have been rolling it out to more and more partners over time," said Ms. Argyilan. "It's been vetted for a while so our ability to execute on it is super high and super strong."