A Line or A-line may refer to:

• A (New York City Subway service), rapid transit line
• A Line (Los Angeles Metro), a light rail line in Los Angeles County, California
• A Line (RTD), commuter rail line between Denver and Aurora, Colorado
• Atlantic Coast Line Railroad, rail line in Florida
• Metro A Line (Minnesota), a bus rapid transit line in Minneapolis, Saint Paul, and Roseville, Minnesota
• A-Line (Hamilton), planned rapid transit line in Hamilton, Ontario, Canada
• RapidRide A Line, bus route in King County, Washington
• Line A (Buenos Aires Underground), Argentina
• RER A, commuter rail line in Paris, France
• A (Los Angeles Railway), former streetcar service

• A-line (clothing), a style of skirt or dress
• Arterial line, a thin catheter inserted into an artery
• A-line, a finding in medical ultrasound of the lung
• A-line bob, one of several bob cut types

• A-A line
• Aline (disambiguation)
• Line A (disambiguation)
• Line (disambiguation)