Cyufjy

Do I need a passport to cross into Canada if I am a born citizen of Canada but I am a permanent resident of the USA with a green card?

To cross into Canada you'll need:

• for land, water, or air travel, a passport, or


• if crossing by land or water, a US passport card (unless you're also a US citizen, you won't be eligible for one), or


• if crossing by land or water, an enhanced driver's license (only a few states provide these, usually at extra cost) (only available to US citizens), or


• for air, water or land travel, a NEXUS trusted traveler card (Canpass will work as well, but this isn't valid for returning to the US)

If the border officer is satisfied with your claim of citizenship, it is possible you will be admitted without these documents, but you're taking a big chance as you may be turned back.

Here is Canada's web page on entry requirements for US citizens and permanent residents.

If you are traveling by land or sea, you can enter both Canada and the US with just your green card.

If you are traveling by air, you will need a passport or NEXUS card.

Entry into Canada:

I have U.S. residency (Green Card). Do I need a visa or an eTA to visit Canada or can I use my Green Card?

...

When travelling by land or sea directly from the U.S., you will only need to provide proof of your U.S. lawful permanent resident status (such as your Green Card).

The omitted material concerns eTA requirements for green card holders, but from the page Dual Canadian citizens need a valid Canadian passport, we can see that Canadian citizens are not eligible for eTA, so that material does not apply to you. To fly to Canada, you need a Canadian passport.

Entry into the US:

What documents do I need to present to reenter the United States?

If seeking to enter the United States after temporary travel abroad, you will need to present a valid, unexpired “green card” (Form I-551, Permanent Resident Card). When arriving at a port of entry, a U.S. Customs and Border Protection Officer will review your permanent resident card and any other identity documents you present, such as a passport, foreign national I.D. card or U.S. Driver’s License, and determine if you can enter the United States. For information pertaining to entry into the United States, see U.S. Customs and Border Protection’s webpage.

The lack of a passport requirement for green card holders is found at 8 CFR 211.2(a)(2):

§ 211.2 Passports.

(a) A passport valid for the bearer's entry into a foreign country at least 60 days beyond the expiration date of his or her immigrant visa shall be presented by each immigrant except an immigrant who:

...

(2) Is entering under the provisions of § 211.1(a)(2) through (a)(7);

Where 8 CFR 211.1(a)(2) reads:

§ 211.1 Visas.

(a) General. Except as provided in paragraph (b)(1) of this section, each arriving alien applying for admission (or boarding the vessel or aircraft on which he or she arrives) into the United States for lawful permanent residence, or as a lawful permanent resident returning to an unrelinquished lawful permanent residence in the United States, shall present one of the following:

...

(2) A valid, unexpired Form I-551, Permanent Resident Card, if seeking readmission after a temporary absence of less than 1 year, ...

Depends on your travel plan. The exact rules (air, land, sea) are spelled out here https://help.cbp.gov/app/answers/detail/a_id/619/~/visiting-the-u.s.---documents-required-for-canadian-citizens-%2F-residents-%2F

In addition to the "formal" rules, US immigration officers have a fair bit of "discretion" so there can be some significant variability there as well.

The easiest and obvious answer: get a passport. It's not that hard or expensive and makes travel quite a bit safer and easier.