In aerospace, thermal applications demand compact, lightweight and efficient heat exchangers. Additive manufacturing processes offer the potential to create highly complex structures that are not achievable through traditional manufacturing methods. This work presents the development of an additively manufactured fluid-fluid heat exchanger that enhances performance, reduces weight and increases compactness compared to a conventional plate heat exchanger. A numerical model of the conventional plate heat exchanger was created, and fluid dynamics simulations with heat transfer were performed. Verification of the simulations was done by experiments. Then a novel heat exchanger was designed using a bottom-up approach and investigated at different levels of complexity using computational fluid dynamics. The internal structure of the final heat exchanger consists of a repeating triple-periodic Schwarz diamond minimum surface elongated in the direction of flow. The heat exchanger was manufactured in the laser powder bed fusion process in AlSi10Mg. It had a 108% higher compactness and 54% lower weight compared to the plate heat exchanger. Numerical analysis yielded the pressure loss was reduced by 50 to 59% while heat transfer was improved by 3 to 5%. Simulation results were validated by experimental assessment of the novel additively manufactured heat exchanger. Further investigations can be conducted to increase compactness and heat transfer by analyzing the minimum partition wall thickness and the impact of wall roughness and deposit formation.