RER may refer to:
- Réseau Express Régional, the commuter rail service serving Paris and its suburbs
- Brussels Regional Express Network (Réseau Express Régional), the commuter rail service serving Brussels and its suburbs and commuters
- Rhône Express Régional, a commuter rail service between Geneva and Bellegarde-sur-Valserine
- Red de Expresos Regionales, a planned commuter rail service in Buenos Aires
- GO Regional Express Rail, a planned commuter rail service in Toronto and its suburbs
- Designation for rebuilt Reading electric multiple units
- Rough endoplasmic reticulum (biology)
- Respiratory exchange ratio
- The Real Estate Roundtable
- RēR, the standard abbreviation for the label Recommended Records
- Resident Evil: Revelations, a Capcom game
- RER Riga, a Latvian football club that played from the 1950s to the 1970s
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