The truth-justification is an enduring explanation for valuing free speech. This paper seeks to advance an account of “assertion”, found in speech act theory, that can identify speech which contributes to truth-discovery in a nuanced way. I apply the dialectic theory of assertion which emphasises the language game of giving and asking for reasons to believe things as assertional social practice. In doing so, I consider what “moves” in this game make sense from a truth-discovery perspective, drawing together contemporary and foundational work on assertion in linguistics to fit the normative context of law. Speech act theory is often invoked in current free speech literature to identify regulable speech, but less typically to identify speech that warrants legal protection. I contextualise my theory within this existing legal work and show how the model I advocate for applies across many formulations of the truth-justification to offer comprehensive explanatory and justificatory power. I also explore the potential appeal of alternative theories of assertion and explain why they are a poor fit in comparison to the dialectical account.