Do I need antivirus on my iMac?

No; instead, avoid downloading software from sources other than the Mac App Store or the developers' websites.

(254034)

Absolutely not. Never ever install the following types of third party apps:

• Antivirus
• Cleaning
• Security
• VPN
• Maintenace

Why, they do NOTHING that Mac OS cannot already do and they all tend to make Mac OS slow, unstable, and appear buggy. In short not only are the damaging they do the exact opposite that you want! All Mac OS needs in order to remain fast, stable and secure is simply keep it up-to-date and restart your Mac about 1X per week.

No.

See also:

Effective defenses against malware and ot… - Apple Community