There isn't a lot of detail in your post, but you make it sound like this is an offer you have on the table, but you haven't started working there yet? If that

There isn't a lot of detail in your post, but you make it sound like this is an offer you have on the table, but you haven't started working there yet? If that is the case, and you aren't 100% happy with the salary, company, location, overall prospects... then you could definitely try to negotiate, but your best course of action is probably to keep looking for another, better offer. With this existing offer, you could ask for more, and you might get it, but you would also risk annoying them and perhaps losing the offer. If you take the job at a salary that isn't great, and later look for something better, then you'll end up with a short term job you'll have to explain in future interviews, and risk being labelled as a job hopper. Once you take a job, negotiating substantial raises is always difficult, it is always easier to negotiate hard before you start.