Background: Cancer-associated thrombosis and enduring inflammation are strongly associated with cancer progression and metastasis. Heparin is the mostly clinically used anticoagulant/antithrombotic drug, and has recently been shown to exhibit antimetastatic and anti-inflammatory activities that are linked to inhibition of P-selectin and/or L-selectin. P-selectin-mediated platelet–tumor cell and tumor cell–endothelium interactions facilitate the initial steps of metastasis. Objectives and Methods: The aim of the present study was to determine the capacity of dermatan sulfates to inhibit P-selectin and to test their potential to affect thrombosis, inflammation and metastasis in respective experimental mouse models. Results: Two dermatan sulfates isolated from the ascidians Styela plicata and Phallusia nigra, composed of the same disaccharide core structure (IdoA2-GalNAc)n, but sulfated at carbon 4 or 6 of the GalNAc, respectively, have opposed heparin cofactor II (HCII) activities and are potent inhibitors of P-selectin. The ascidian dermatan sulfates effectively attenuated metastasis of both MC-38 colon carcinoma and B16-BL6 melanoma cells and the infiltration of inflammatory cells in a thioglycollate peritonitis mouse model. Moreover, both glycosaminoglycans reduced thrombus size in an FeCl3-induced arterial thrombosis model, irrespective of their HCII activities. The analysis of arterial thrombi demonstrated markedly reduced platelet deposition after dermatan sulfate treatment, suggesting that the glycosaminoglycan inhibited P-selectin and thereby the binding of activated platelets during thrombus formation. Conclusions: Collectively, these findings provide evidence that specific inhibition of P-selectin represents a potential therapeutic target in thrombosis, inflammation and metastasis, and that ascidian dermatan sulfates may serve as antiselectin agents.