In this paper, I begin a philosophical theorisation of the phenomenon of toxic positivity (TP) within the framework of social epistemology. TP is the phenomenon of people being positive and optimistic to a degree that is unreasonable in a given situation, and as such makes others feel as if their own (less than positive) feelings are invalid or in some way wrong. I begin by providing an example of TP. I then identify four features of TP: appropriate emotion, unreasonableness, dismissal, and potential harm. Following this, I discuss the possible epistemic effects of TP and argue that it can affect knowledge in three ways: doubting belief, losing belief, and undermining self-trust. Finally, I argue that TP can in some cases be a form of gaslighting and can undermine epistemic agency, both of which are epistemic injustices.