| Reginald I, Count of Bar | |
| Noble Family: | House of Montbéliard |
| Father: | Theodoric I, Count of Montbéliard |
| Issue: | Reginald II, Count of Bar |
| Mother: | Ermentrude of Bourgogne |
| Spouse: | Gisele of Vaudémont |
| Death Place: | Mediterranean Sea |
Reginald I (also called "the One-eyed", Reinald I, Renaud I; - 10 March 1149) was Count of Bar (1105–1149). Barrois, during the Middle Ages, was the territory of the counts and dukes of Bar, in the eastern part of present-day France, bordering Lorraine.
He was the son of Theodoric I, Count of Montbéliard and Ermentrude of Bourgogne, the daughter of William I, Count of Burgundy.
Reginald's first wife is unknown. He later married Gisele de Vaudémont, widow of Rainard III, Count of Toul, and daughter of Gérard I, Count of Vaudémont, and his wife Heilwig von Egisheim. Reginald and Gisele had eight children:
Reginald was one of the leaders of the Second Crusade in 1145. He was drowned somewhere in the Mediterranean Sea on his return voyage to Europe on or before 10 March 1149.
Reginald I of Bar succeeded Theodoric II of Bar (r. 1092–1105) who was succeeded in turn by Reginald II of Bar (r. 1150–1170) in 1150.