Last month, Heap accidentally struck and killed Holly -- who would have turned 4 today -- while moving his truck.
"We're with you, Ravens. #HugsFromHollyDay," wrote the Steelers in a retweet of the Ravens requesting followers to "Give a hug. Wear pink. Spread some kindness."
In addition to the Steelers, dozens of teams and several players tweeted support to the Ravens and Heap, who spent 10 of his 12 NFL seasons with Baltimore.
The hugsfromholly.com site encourages people to wear pink in Holly's honor, share hugs and spread love through random acts of heartfelt kindness in the community, and spread joy through the #hugsfromhollyday hashtag alongside happy family photos.
"Holly was known to give the best hugs, and her love for everyone and everything in life was contagious," according to the site, which also provides a link to the Baltimore Community Foundation for donations in Holly's name. "Let's spread this joy as we scatter sunshine in Holly's honor on her birthday."
The Steelers and Ravens play one of the NFL's fiercest rivalries, but they've been known to support each other in difficult times off the field. The Ravens issued several statements honoring Steelers owner Dan Rooney, who died last month at age 84. "Dan Rooney was the conscience of the NFL while serving as one of the powerful influences recently and in the history of our league," owner Steve Bisciotti said in a statement. "He was kind, classy, and a true gentleman. He was a giant in our industry."
Several Steelers tweeted support in March 2016 after former Ravens cornerback Tray Walker died in a dirt bike crash. "Saddened by the passing of Tray Walker, a member of our NFL family," coach Mike Tomlin tweeted. "Thoughts and prayers with his family and the Ravens organization."
ESPN's Jamison Hensley contributed to this report.