If you see something at Walmart, do not post it on People of Walmart, call Homeland Security instead.

The Homeland Security Secretary Janet Napolitano announced the unusual alliance Monday on the department’s blog and in a promotional video posted to YouTube.

“Homeland security starts with hometown security, and each of us plays a critical role in keeping our country and communities safe,” she said. “This partnership will help millions of shoppers across the nation identify and report indicators of terrorism, crime and other threats to law enforcement authorities.”

At least 200 Wal-Mart stores will roll out security announcements within 24 hours, Wal-Mart spokesman Dan Fogleman said. By month’s end, 588 stores in 27 states will be participating in the program. A short video featuring Napolitano will appear on TV screens at select checkout lanes, asking Wal-Mart shoppers to contact local law enforcement to report suspicious activity.

“If you see something suspicious in the parking lot or in the store, say something immediately,” Napolitano said in the video. “Report suspicious activity to your local police or sheriff. If you need help ask a Wal-Mart manager for assistance.”

Fogelman said employees won’t receive any special training as part of the program. But Wal-Mart managers do work closely with local police, he said.

“We work with local law enforcement all the time,” Fogleman said. “If someone needs help, we will certainly assist. If someone asks us to call police, we will call police.”

“If You See Something, Say Something” has become a familiar sight for New York subway riders since 2002. The campaign was created by The New York City’s Metropolitan Transit Authority and funded, in part, by $13 million from a Homeland Security grant program.

The Homeland Security Department has subsequently expanded the program to other transit agencies and some private entities, including the Mall of America, the American Hotel & Lodging Association and sports and general aviation industries.

But some marketing experts say they aren’t sure how the message will play in a retail setting.

While the message may be familiar one for people who use public transportation and airports, it’s new one for many in smaller hometowns, said Sarah Kerkian, insights supervisor at Cone Inc., a strategy and communications firm in Boston.

“It may make shoppers feel cared for,” Kerkian said. “But others may say, ‘Who are you to bring me this message?'”

But Fogleman, the Wal-Mart spokesman, said the Homeland Security alliance with the nation’s largest retailer makes perfect sense

“Wal-Mart is a place where people gather, it’s a place where you sometimes see your neighbor the most,” Fogleman said. “It is important to Wal-Mart that it help protect the safety of its community.”