Robert Cooper Grier (March 5, 1794-September 25, 1870), was an American jurist.

Grier was born in Cumberland County, Pennsylvania to a Presbyterian minister, who tutored him until he entered Dickinson College. Grier graduated from Dickinson in only one year, and remained there as an instructor until taking a position at a school his father ran. He succeeded his father as headmaster in 1815.

While a teacher, Grier read law on his own time, and passed the bar in 1817, at which time he entered private practice. Grier married Isabelle Rose in 1829 and the couple had one child.

Grier was a political organizer for the Jacksonian Democrats, and in 1833 Grier was given a patronage appointment to a state district court, newly created for him. He served there for 13 years, developing a reputation for competence.

Justice Henry Baldwin died in 1844 during the administration of President Tyler. Tyler made two attempted appointments to the seat, both rejected, and seat remained open when James K. Polk ascended to the presidency. Polk also made two nominations, one of whom refused the appointment (future President James Buchanan) and one of whom was not confirmed by the Senate. Polk finally nominated Grier, plucking him from relative obscurity. Grier was unanimously approved by the Senate, and joined a fellow Dickinson alumnus, Roger Brooke Taney, on the Court.

Grier served on the court until 1870, at which point he was quite frail, having suffered three strokes in 1867. His participation on the court was extremely limited by the end of his term, and he retired only after his colleagues pressed him to do so. He died a year later.

Grier was the lone northerner to side with the majority in the Dred Scott decision, though after the outbreak of the Civil War he supported the Union. Grier also wrote the opinion on the Prize Cases , which declared Lincoln's blockade of Southern ports constitutional.

Data drawn in part from the Supreme Court Historical Society and Oyez.