"They have a cave troll," Monaghan captioned a photo on Instagram of the group recreating a scene from the Mines of Moria in The Fellowship of the Ring.
"When we reunite in a pub and someone says 'they have a cave troll' ...#squadgoals," Bloom added on his own account.
Monaghan, who played Meriadoc Brandybuck, also shared a picture of himself with Mortensen, who portrayed Aragorn, and with Wood and Boyd, who played Frodo Baggins and Peregrin Took, respectively.
"Doffing the off the shoulder band look with aplomb @boydbilly making @electrice and I look like onions," he wrote.
The Lord of the Rings movies -- The Fellowship of the Ring, The Two Towers and The Return of the King -- are based on the J.R.R. Tolkien book series of the same name, and earned over $2.9 billion at the box office.
"I remember being there two months out from shooting and just being surrounded by this positive energy of, 'We're a part of something really special,'" Wood told The Huffington Post in December.
"It was the most extraordinary experience of my life," he added. 'It's a wild thing to look back on. I feel like I kind of grew into being a man throughout that experience and had more responsibility than I'ever ever had and lived away from home longer than I've ever had."
The Fellowship of the Ring, which co-starred Ian McKellen as Gandalf, Sean Bean as Boromir and John Rhys-Davies as Gimli, celebrated its 15-year anniversary in December. Wood and Bloom also played Frodo and Legolas in The Hobbit movies.