The most important thing is to get your kitty to a vet to make sure s/he doesn’t have a blockage of some sort. Male cats, for example, can get crystals in their urinary tract and can die from it if not treated immediately.
UTI (Urinary Tract Infection) or a blockage – Sometimes if they have a UTI it will make them “go” outside the box because they associate the box with pain when urinating. If they are peeing on the couch, for example, it probably feels softer. Take your kitty to the vet to get checked out.
Bladder Infection – S/he may have a bladder infection. If it’s painful when s/he pees, s/he’ll associate the litter box with pain and pee somewhere else. Take your kitty to the vet just to be sure s/he’s healthy. The first step is always to take your kitty to the vet to make sure it’s nothing medical.
Hormonal – if your kitty isn’t spayed or neutered. Some females will urinate to “advertise” that they are in heat. And males will spray to mark their territory. Spraying is quite different than just peeing.
Message – The cat is trying to “tell” you something – they need an additional litterbox, they don’t like where their current boxes are located, they don’t like the kind of litter, they don’t like the kind of box (some cats do NOT like hooded or automatic boxes, for example).
Territorial – It could be a territorial behavior with another animal in your house or even one that visits just outside of the home. This could also happen if the kitty is required to share the box with another cat.
Dirty Boxes – How often do you scoop your litter box? It should be once a day and sometimes twice a day. You should also completely clean and replace litter about once a month. Try cleaning the box more often too.