Best answer: Yes, but only when taking careful precautions. A GPU positioned vertically too close to a side panel will restrict airflow in the case and suffer raised temperatures. You'll need plenty of breathing room or liquid cooling to accommodate the vertical GPU.
Why mount a GPU vertically?
The most common reason is to prevent sagging with heavier cards pulling so much weight down on the PCIe slot. The secondary is to fully appreciate RGB lighting built into some cards, which otherwise can be obstructed by surrounding components. It's popular in the custom PC building scene due to putting the GPU on full display via a transparent side panel, and after all, when you've paid a pretty penny for your high-end graphics card, it makes sense to show it off.
It's not something to jump into without serious research since you'll lose access to surrounding PCI lanes and often make nearby M.2 SSD sockets harder to reach. If vertically mounting a GPU with care, the results can be stylish and make more room for a fancy liquid-cooling setup. If you're only looking to prevent GPU sagging, you might be better off picking up a GPU support brace instead.
How do I mount it?
You'll need to choose one of the best PC cases for vertical mounting or likely suffer from restricted airflow and choke the GPU fans against the side panel. If your setup is liquid cooled, airflow will be less of an issue, but it doesn't hurt to have some clearance between the GPU and side panel with a large case. If you have the space, you'll need a kit to mount the graphics card and check that the riser cable is compatible with your PCIe lane version. Most are PCIe 3.0, but 4.0 riser cables are available.
Remember that not every PC case is compatible with a vertical GPU mount simply because they're big and spacious, so you'll need to check that it has somewhere to properly attach a mounting plate and riser card. It's not a task for a novice builder, but the results are certainly pretty. If you're ready for the challenge, you can check out some vertical mounting brackets from trusted companies like Cooler Master.
Our pick
Cooler Master vertical GPU holder kit V2
Let's get vertical
Compatible with most ATX cases and adjustable to fit your GPU position, this V2 kit includes a PCIe 3.0 riser cable. Its high-quality construction is ready to safely support your graphics card and display it vertically.