SAN FRANCISCO, CA - NOVEMBER 03: A McRib is seen at a McDonald's restaurant on November 3, 2010 in San Francisco, California. The sandwich arrived on the menu for the first time since 1994 and is offered at all McDonald's nationwide for a limited time until December 5, 2010. The McRib was first introduced in 1981. (Photo by David Paul Morris/Getty Images)

McDonald's McRib fans may have to hunt harder than usual for the popular sandwich this year.

It will only be available at participating restaurants, rather than nationwide, the company said.

"We're letting the restaurants determine if this menu item is one their customers will enjoy," wrote McDonald's spokeswoman Lisa McComb in an email.

While McComb declined to say when the boneless barbecue pork sandwich would be available, one North Dakota franchise told CNBC it would begin selling the sandwich Monday.

"The McRib always makes a grand entrance on our menus and we don't want to steal its thunder—so stay tuned," McComb said.

Abroad, McDonald's units in Philippines and Guatemala have been advertising the sandwich's availability via social media.

The McRib is the rare U.S. limited-time offering in the fast-food industry that returns annually to great fanfare. Germany is the only country where the McRib is sold year-round.

Introduced in 1982, the McRib has become so popular that it's even sparked one fan site devoted to spotting sightings of it. This year, McDonald's will be looking for the regular draw of the McRib attraction to boost U.S. sales.

Performance at its U.S. unit has floundered with comparable sales dropping 1.5 percent in the second quarter. These negative sales trends persisted into July and August.

As it seeks to boost sales, McDonald's ramped up its campaign this week to educate consumers about its food with a series of videos aimed at dispelling rumors about its meat.