United States Postal Service Riderwood, MD 21139 Phone : (410) 821-7502 Business Category Post Office - Riderwood 8016 Bellona Ave Riderwood, MD 21139 Phone : (410) 821-7502 Post Office - Riderwood Business Category Post Offices Toll-Free           (800) ASK-USPS Fax           (410) 823-7072