Invasive plants may cause significant environmental risks by affecting biological diversity, ecosystem services, agriculture, and forestry. Safe utilization of invasive plant biomass by obtaining new outcome products useful for bioeconomics is a challenging but promising solution. Hydrothermal carbonization (HTC) presents a sustainable and cost-effective approach to transforming invasive plant biomass into new products while simultaneously supporting carbon capture aims. This study utilized invasive plants such as lupine [Lupinus polyphyllus Lindl.], Sosnowsky’s hogweed [Heracleum sosnowskyi Manden.], and Japanese knotweed [Polygonum cuspidatum Siebold & Zucc.] as biomass sources for HTC to produce artificial humic substances (AHS). The greatest impact on the yield of AHS (maximum gained yield 62%) has the duration of the HTC process (up to 6 h) and temperature of the treatment (from 160 up to 250 °C), as well as the catalyst used (an alkaline medium is preferable). During the HTC treatment, significant changes in the invasive plant biomass composition occur as indicated by the removal of labile components of organic matter. Obtained AHS are essentially similar to natural humic matter and have biostimulatory properties; thus, they can find application in agriculture and other areas of bioeconomics.