As of April 26, 2022, lawful permanent residents of the U.S. must show the following documents for all methods of travel to Canada:
- a valid passport from their country of nationality (or an equivalent acceptable travel document) and
- a valid green card (or equivalent valid proof of status in the United States)
You need an official proof of status as a lawful permanent resident of the U.S., such as one of the following:
- valid permanent resident card (Form I-551)
- foreign passport with an unexpired temporary I-551 stamp (also known as an Alien Documentation, Identification and Telecommunication [ADIT] stamp)
- foreign passport with a temporary I-551 printed notation (“Upon endorsement serves as temporary I-551 evidencing permanent residence for 1 year”) on a machine-readable immigrant visa upon endorsement with a U.S. Customs and Border Protection admission stamp
- expired permanent resident card (Form I-551) with Form I-797 (Notice of Action) for pending Form I-751 (Petition to Remove Conditions on Residence) or Form I-829 (Petition by Investor to Remove Conditions on Permanent Resident Status)
- expired permanent resident card (Form I-551) with Form I-797 (Notice of Action) for pending Form I-90 (Application to Replace Permanent Resident Card [green card])
- valid re-entry permit (Form I-327)
- Form I-94 with an unexpired temporary I-551 stamp (ADIT stamp) and a passport-style photo
Source: https://www.cic.gc.ca/english/helpcentre/answer.asp?qnum=1053&top=16