I am not sure whether the following sentence is grammatically correct. Could someone please guide me on this? Is there a better alternative?
I am the new CEO of this company who has taken over from Josh.
"Who" is a pronoun for the subject, and "I" is the subject of this sentence, so yes the sentence is grammatically correct.
That said, I think it reads just a tiny bit awkwardly and better alternatives might be:
I've been trying to figure out what exactly I find awkward about the original, and I think it's because I experienced a moment of ambiguous interpretation between "I am the new CEO [of this company] who has taken over from Josh." and "I am the new CEO of [this company who has taken over from Josh]." Obviously a company is not a person and we probably wouldn't use "who" for it, so it resolves in the end.