How to turn off screen rotation in Ubuntu 18.04 LTS?

The best solution I have found so far is to use the icon that is available in the top right corner of the screen, when you click on the down arrow. The icon is the one in red in the screenshot below:

It might also look like this:

If you want to remove this feature then fire below commands :

Try this command to stop and disable

$ sudo systemctl stop iio-sensor-proxy.service
$ sudo systemctl disable iio-sensor-proxy.service

Try this command to remove

$ sudo apt-get remove iio-sensor-proxy

gsettings set org.gnome.settings-daemon.plugins.orientation active false

The above just worked for me. Seems the simplest solution

You may use the keyboard combination Super+O to toggle the rotation lock ON/OFF as described in @Sylvain's answer.

Note that Super is usually the key with Windows logo.

What do you mean "turn off screen rotation" ? Do you mean go back to normal orientation ?

Anyway, in the new gnome-control-center of gnome 3.28, in Devices > Screen Display tab, you can click on Orientation and a menu appears to let you choose between

- Landscape
- Portrait right
- Portrait left
- Landscape (flipped)

I guess this is what you want ?

for me none of these work. As soon as I put the computer to sleep and wake it up again, it puts itself in the wrong orientation (due to the fact that the computer is sitting on a tilted support that raises it up). The only thing that I manage to do quickly is

xrandr -o normal

to fall back to normal orientation. This is painful

Use the code given below and it will work.

gsettings set org.gnome.settings-daemon.peripherals.touchscreen orientation-lock true
gsettings set org.gnome.settings-daemon.plugins.orientation active false