Could aliens look completely different from humans?

Yes. Alien life could look completely different from humans, and it would be reasonable to expect that most extraterrestrial life would not resemble us.

We do not yet know what extraterrestrial life looks like, so any answer is speculative and based on what we know from Earth biology and astrobiology. An organism’s shape would likely depend on its environment, chemistry, and evolutionary history. For example, much stronger gravity could favor short, broad, heavily muscled bodies; a world covered mostly by water might favor streamlined aquatic forms; and a planet around a dim red dwarf star could produce organisms with very different sensory systems, such as large eyes, infrared sensitivity, or even no eyes at all if another sense is more useful. Evolution is also shaped by chance, so there is no reason to expect intelligence or complex life to take a humanoid form.

There are a few reasons alien life might still share some features with Earth life:

• Convergent evolution can produce similar solutions to similar problems, like eyes, wings, or grasping limbs.
• If a world has comparable conditions to Earth, some traits might recur even if the overall body plan is very different.

So the short answer is: yes, aliens could look nothing like humans, though some features might still be similar if they solve similar survival problems.