Change the door knobs. You can change them back when you move.

Don't know if there is anything like it still made, but way back when, there was the Weiserbolt, a doorknob that could extend into a deadbolt.

https://i.imgur.com/q1uIkPM.jpg

OP, maybe you could find one on Ebay and, as suggested, replace the lockset with it.

Since you haven't bothered to provide any information about what is currently on the door: a Schlage 2 key deadbolt is your answer.

Your lease almost certainly requires that the landlord can get access, so you will need to give them a key.

Double keyed deadbolt.

Double sided deadbolts are dangerous if there’s a fire

This. When we sold our last house I had to remove the double keyed deadbolt from the basement door. It took all of 5 minutes. The inspectors saw it and they buyers wanted it removed. On the house we bought the inspectors saw it and we decided we just wouldn't use the deadbolt and left it.

This is the correct answer. There is no other magical locking device that can lock from the outside without some sort of alterations, to the door or otherwise. (barring some rube goldbergesque contraption)

Alternatively, ask your landlord to install a deadbolt. If you don't feel safe where you are, leave.

don't think they are talking about that

Why would anyone need this? Is this even safe?

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Oh absolutely get that. I meant a double lock. Why would you want to have a key to open an inside door to get out? I’m in Canada and I’ve never seen This. Seems like a serial killers dream.

What do you mean “only lock from the inside”? Are you looking for a lock that needs a key on both sides, or one with a handle on both sides?

Because if it’s with a key on both sides, that’s SUPER dangerous if there’s a fire or other reason you need to get outside quickly. Imagine trying to rush out of your house and not being able to find the key.

If you’re asking for a handle on both sides, what’s the point of having a lock at all?

I’m confused.

So someone can't smash a window in the door and then reach in and unlock it.

Technically, not safe. In the event of a fire, nobody has time to look for a key. However, I used one of these for my back door when I had a pool and little kids that I didn't want wandering into the backyard unsupervised.

There's a balance. I had one on my backdoor to keep my kids from getting to the backyard pool.

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I mean a simple lock for my bedroom. Maybe I should have clarified that 😅

I can see that. I was looking at the plastic child resistant latches for the lever door handles - an adult would probably be strong enough to break it off for an emergency exit.

I’ve seen these used on doors that have glass. Deadbolts are fairly useless if you can smash the glass and just unlock the door.

Okay, so from the comments you're renting a house and have house mates you don't fully trust. Your bedroom does not have a lock that you can lock when you leave.

The best way would be to replace the bedroom door knob with a keyed entry door knob. Most bedroom knobs only lock from the inside and can be popped really easily with an Allen wrench.

A keyed entry set usually is used on an outside door and you would lock it on the outside with a key and a turn on the inside knob.

If you have a standard door it is super easy to swap. I'd talk to your landlord and get permission to swap the handle in writing.

Leave the key in the inside lock when you're home.

Yes here, they are also used for safety, say you have a door that opens to a drop off because the deck is not finished yet

They make a deadlock that has a key on both sides.

https://www.amazon.com/dp/B001172NJM

Ask your landlord for permission.

Get a keyed entry doorknob and replace the existing doorknob.

It should take a screwdriver and maybe 10 minutes.

Just swap the door knob to one with what you like. If you can get one that matches what is there except it had the lock. Will make it less likely anyone else notices. Save the old one and switch it back before you leave.