Just to expand on this, using flask application config just as you've mentioned, you can set everything except when setting SESSION_COOKIE_SAMESITE=None Google Chrome doesn't seem to place the value as "None", which then defaults to "Lax".

How i worked around this problem was to add the cookie back into the response header. First I had to get the cookie value because using request.cookies.get("my_cookie") doesn't seem to extract the cookie value from the response and always appears as None.

secondly, using the response.set_cookie() still doesn't set the samesite=None value. I have no idea why because i'm using the latest version of flask and Werkzeug which apparently should fix the problem but it doesn't. After lots of testing, I found out using the response.headers.add() works to add a Set-Cookie: header but I needed a way to extract the cookie value to ensure I can get the same session. After looking through flask docs and other online forums. I found out that I can actually call SecureCookieSessionInterface class and get the signed session from there.

from flask import session
from flask.sessions import SecureCookieSessionInterface

# where `app` is your Flask Application name.
session_cookie = SecureCookieSessionInterface().get_signing_serializer(app)

Lastly, i had to ensure that the same session is added to the response after the request has been established rather than calling it on every route which doesn't seem feasible within a full fledged application. This is done by using the after_request decorator which runs automatically after a request.

@app.after_request
def cookies(response):
    same_cookie = session_cookie.dumps(dict(session))
    response.headers.add("Set-Cookie", f"my_cookie={same_cookie}; Secure; HttpOnly; SameSite=None; Path=/;")
    return response

What I noticed in Chrome is that, it basically sets a duplicate cookie with the same signed value. Since both are identical with one having samesite=None in the response header and the other blocked by Chrome seems to be ignored. Thus, the session is validated with the flask app and access is allowed.

From: https://github.com/GoogleChromeLabs/samesite-examples/blob/master/python-flask.md

Assuming you're on the latest version of werkzeug that includes the fix to this issue, you should be able to use set_cookie() like this:

from flask import Flask, make_response

app = Flask(__name__)

@app.route('/')
def hello_world():
    resp = make_response('Hello, World!');
    resp.set_cookie('same-site-cookie', 'foo', samesite='Lax');
    resp.set_cookie('cross-site-cookie', 'bar', samesite='None', secure=True);
    return resp

Otherwise, you can still set the header explicitly:

from flask import Flask, make_response

app = Flask(__name__)

@app.route('/')
def hello_world():
    resp = make_response('Hello, World!');
    resp.set_cookie('same-site-cookie', 'foo', samesite='Lax');
    # Ensure you use "add" to not overwrite existing cookie headers
    resp.headers.add('Set-Cookie','cross-site-cookie=bar; SameSite=None; Secure')
    return resp

A mistake easily made (as I did) is to confuse None with "None". Be sure to use the string instead of the python literal like so:

response.set_cookie("key", value, ..., samesite="None")

samesite=None would indeed be ignored and defaults to "Lax".

I had a similar issue where it kept setting SESSION_COOKIE_SAMESITE to Lax. In my case it was caused by flask-talisman, who was overwriting my config. Specifying

session_cookie_samesite=app.config["SESSION_COOKIE_SAMESITE"]

in the Talisman constructor worked.