Both cactuses and cacti are correct plurals of cactus. That surprises many writers, because people often assume only cacti is proper. In standard English, both forms are widely accepted.
Why are there two plurals? The word cactus came into English from Latin. In Latin, a common plural pattern turns words ending in us into i, which gives us cacti. But once a word becomes part of English, it can also take a regular English plural ending, which gives us cactuses.
This is not unusual. English keeps some older plural patterns while also creating newer ones that feel more natural to modern speakers. As a result, both versions can sound correct, depending on context, region, or style.
- Cactuses: The garden center sells small cactuses in clay pots.
- Cacti: Several rare cacti grow in the desert greenhouse.
If you want a simple rule, this works well: use the form that matches your audience and keep it consistent in the same piece of writing. In everyday English, either choice is fine.