The Baggies were leading 2-1 at Stamford Bridge when Reid was adjudged to have fouled Ramires in the penalty area, and Marriner had no hesitation pointing to the spot.
"For me, it was very soft. There was obviously contact there but it was a fair shoulder-to-shoulder challenge and Ramires has gone to ground very easily," Reid told Sky Sports News.
"Going into the dying seconds of the game leading 2-1 and to then concede a penalty - we all feel very hard done by."
West Brom are ninth in the Premier League following their draw with the Blues.