memory for imac 27" late 2011

Amazon, OWC, Newegg, there are many DIMM vendors.

I’ve purchased DIMMs for Mac most recently from OWC, and have had no issues.

AFAIK, there’a no iMac 27” Late 2011 model, though there is a mid 2011. The iMac late 2011 appears to (only) be a 21.5” model.

If this is the iMac 27” mid-201, there are four slots for 204-pin PC3-10600 (1333 MHz) DDR3 SO-DIMM memory sticks, and with some installation constraints. Usual install is in pairs. Details on iMac memory upgrades, including this one: Install memory in an iMac - Apple Support

If this iMac still has a hard disk drive, an SSD upgrade may or will be a bigger performance boost than a memory upgrade, and should be seriously considered here.

So while you are browsing the OWC website, look for their SSD upgrades for your Mac.

I have a Late 2009 27" iMac, and the SSD I installed has kept that iMac as the Mac I use at work every work day. Yes, I've also upgraded the RAM, but it is the SSD that made the biggest difference and has kept this Mac functional. Mine is stuck at High Sierra, and I'll most likely replace it after High Sierra is no longer getting security updates (which is most likely in 16 months after the 10.16 release comes out in the Fall of 2020). But even then, I'm thinking I'll put my iMac into "Target Display Mode" and still use the iMac as an external monitor. I've already got the USB-C to miniDisplay Port cable. 😀

https://eshop.macsales.com/upgrades/imac-27-inch-mid-2011-2.7-ghz/memory

Ah, didn't catch the Late part, thanks.