Prioritizr is an R package for solving systematic conservation prioritization problems using integer linear programming (ILP) techniques. The package offers a flexible interface for creating conservation problems using a range of different objectives and constraints that can be tailored to the specific needs of the conservation planner. Conservation problems can be solved using a variety of commercial and open-source exact algorithm solvers. In contrast to the algorithms conventionally used to solve conservation problems, such as greedy heuristics or simulated annealing, the exact algorithms used by prioritizr are guaranteed to find optimal solutions. This package also has the functionality to read Marxan input data and find much cheaper solutions in a much shorter period of time than Marxan (Beyer et al. 2016). Check out the prioritizrshiny R package to interactively build and customize conservation planning problems.