Maraging steel C250 and its low cost, Co-free version T250 have been widely applied in industry for various applications requiring high strength and high toughness. However, one major drawback of this steel is the relatively poor corrosion and stress corrosion cracking (SCC) resistances. The cost of steel C250 is also high due to high Ni, Mo and Co contents. In this sense, high strength stainless steel has undisputed advantage. From an attainable strength stand point; precipitation hardening martensitic stainless steel is in the category of stainless steels that can compete with high strength maraging steels such as C250 and T250. However, existing commercial martensitic PH stainless steels still cannot fully compete with C250 and T250. Some PH stainless steels such as 17-4PH, 15-5PH and PH13-8Mo have much lower strength and the toughness of others such as XPH12-9Mo, Vasco734 and C455 is inferior to that of maraging steels at an equivalent strength level. Recently, Carpenter Technology has developed a new stainless steel C465 that was reported to have a similar strength – toughness combination as C250 maraging steel. However, C465 requires elaborated cryogenic treatment after solution treatment and some aspects, especially corrosion and SCC resistances of this steel could be further improved. An extensive research program was initiated at Allvac with the aim of developing a new stainless steel with ultra-high strength, high toughness and excellent corrosion / SCC resistance. This steel is processing-friendly without using tightly controlled heat treatment and elaborated cryogenic treatment. The roles of major hardening elements Al, Ti and Cu in this type of steel were thoroughly investigated and the optimum levels of these three elements was determined to guarantee the best strength –toughness combination. Ni and Mo levels were carefully balanced to ensure a high toughness and better corrosion/SCC resistance. In order to further increase corrosion/SCC resistance W was added in this steel. This alloy can be solution-treated at 1700°F – 1800°F and aged at 950°F – 1050°F (H950 – H1050), depending on property requirements. No cryogenic treatment is required after solution treatment. The yield strength of this steel is 230 ksi or higher at H950 condition and 220 ksi or higher at H1000 condition with fracture toughness greater than 90 ksi.in1/2 for H950 and 105 ksi.in1/2 for H1000 condition. Fatigue strength of this steel is 100 ksi or higher at both H950 and H1000 conditions. This steel has excellent pitting/crevice corrosion resistance and no crack propagation was observed in KISCC tests in 3.5% NaCl aqueous solution at an applied KI of 80% KIC for up to 1000 hrs. Allvac S240TM can be used in many applications requiring high strength, toughness and excellent corrosion / SCC resistance.