Hemp seeds are currently used mainly for oil extraction, generating waste that could be potentially exploited further as a source of proteins and other bioactives. This study aims to valorise hemp waste (Cannabis sativa, L.) from previous oil extraction as a source of protein by analysing the effect of high-pressure processing (HPP) pre-treatments (0–600 MPa; 4–8 min) combined with conventional or ultrasound-assisted extraction (UAE) methods on protein recovery/purity, amino acid composition, and protein structure. Overall, maximum protein recovery (≈62%) was achieved with HPP (200 MPa, 8 min) with UAE. The highest protein purity (≈76%) was achieved with HPP (200 MPa, 4 min) with UAE. Overall, UAE improved the extraction of all amino acids compared to conventional extraction independently of HPP pre-treatments. Arg/Lys ratios of the protein isolates ranged between 3.78 and 5.34, higher than other vegetable protein sources. SDS-PAGE did not show visible differences amongst the protein isolates. These results seem to indicate the advantages of the use of UAE for protein recovery in the food industry and the need for further studies to optimise HPP/UAE for an accurate estimation of processing costs and their effects on the composition and structure of proteins to contribute further to the circular economy.