Black Hill Regional Park Dog Park - Boyds, MD
Cabin John Dog Park - Bethesda, MD
Olney Manor Dog Park - Olney, MD
Ridge Road Dog Park - Germantown, MD
Wheaton Dog Park - Wheaton, MD
Another option is the Rockville City run dog park at King Farm. Click here for more details.
True story: I went to Cabin John Dog Park with my 2 dogs when I saw an old man leashing up my little Yorkie. I approached the man and said, "Um, hi. That's my dog you just leashed you know." He just looked at me and smiled, and started to walk away. I stopped him again and said, "No, that's MY dog. See? It's a girl. Your yorkie is right there...the boy dog." Horrified, he just looked at me and apologized profusely...haha...Mia almost got dognapped! =P