Additive manufacturing (AM), also known as 3D printing, is a revolutionary manufacturing technology that has attracted many industries in the past two decades. This is because AM enables the manufacturing of complex shaped geometries without the limitations of other manufacturing techniques. In this paper, the design, development and testing of additively manufactured, monolithic catalyst beds are described. A novel design methodology was employed and achieved catalyst bed designs with complex geometry and high geometrical surface area whilst achieving an acceptable pressure drop. Catalyst bed samples incorporating alumina ceramic lattices with strut diameters ranging from 0.15 to 0.30 mm were fabricated via AM and a subsequent heat treatment. The surface areas of the samples were improved using different wash coats, including the use of gamma alumina and a mixture of gamma alumina and carbon nanotubes (CNT). Manganese oxides were used to coat the catalyst bed and decompose hydrogen peroxide. Four full-scale catalyst beds with the most promising candidate geometries and wash coats were then manufactured and subsequently tested in a 20 N-class HTP (High Test Peroxide) monopropellant thruster. The firing results show that the additively manufactured catalyst beds generally outperformed the baseline catalyst bed containing ceria pellets that were also coated with manganese oxides.