By taking the collective character of scientific research seriously, some philosophers have claimed that scientific knowledge is indeed collective knowledge. However, there is little clarity on what exactly is meant by collective knowledge. In this article, I argue that there are two notions of collective knowledge that have not been well distinguished: irreducibly collective knowledge (ICK) and jointly committed knowledge (JCK). The two notions provide different conditions under which it is justified to ascribe knowledge to a group. It is argued that ICK and JCK need to be approached independently, each of which can shed light on different aspects of science, knowledge production, and acceptance.