We exhaustively classify the Lie reductions of the real dispersionless Nizhnik equationto partial differential equations in two independent variablesand to ordinary differential equations. Lie and point symmetries of reduced equations are comprehensively studied,including the analysis of which of them correspond to hidden symmetries of the original equation. If necessary, associated Lie reductions of a nonlinear Lax representationof the dispersionless Nizhnik equation are carried out as well. As a result, we construct wide families of new invariant solutions of this equationin explicit form in terms of elementary, Lambert and hypergeometric functionsas well as in parametric or implicit form. We show that Lie reductions to algebraic equations lead tono new solutions of this equation in addition to the constructed ones. Multiplicative separation of variables is used for illustrative constructionof non-invariant solutions.