Lisbon in January?

Can't speak to the weather, but I was just there in May. Very crowded and touristy! I think I might have preferred going off season.

And you need not stick to Lisbon. Take a train or bus somewhere else for a few nights, like Porto. See if you can get an open jaw "multi-city" flight (one ticket, not two one-ways) into Porto out of Lisbon or vice versa. If you can't get a good fare into or out of Porto, I'd advise training immediately north to Porto after you land in Lisbon and come back by train and spend the last 4-5 nights in Lisbon. Or plan day trips from Lisbon.

google "weather averages in January in Lisbon Portugal"
It depends on what you are use to. It would be nice break from Chicago in January for example.

https://www.holiday-weather.com/lisbon/averages/january/

We were there in Feb this year and ate out out side alot. Then a friend was there in Alril and had heavy rain, so it just depends. Beautiful city and lovely people.

I echo the comment about Portugal being SO much better than Chicago in January. The biggest issue is rain, but we have lovely days in the winter when outside lunches are common. The skies are clear compared to summer (haze), and there are no tourist hordes.
Remind yourself that it never freezes - from central Portugal south. And, when it rains, it’s rarely an all day event. Daytime temps are usually high 50’s to low 60’s. Night temps sometimes dip into high 30’s but are usually low 50’s to high 40’s.
The only caveat is heat in apartments. If you stay in a hotel, there should be no problem. But, if you choose a vacation rental, be sure to ask how it is heated. Since it’s not that cold, most apartments don’t have regular heat like in the States.. The joke is that our apartments are usually colder than outside!