The Martingale system is perfectly legal. Many online casino players use it to determine how much to stake on each hand or spin of the wheel. You can also use the Martingale system in sports betting., but a horse racing Martingale system would be difficult.
The use of betting systems like the Martingale is generally allowed in casinos, as long as they are not being used to manipulate the outcome of the game. However, it is important to keep in mind that this system is not foolproof and can actually lead to significant losses if you have a long streak of losses.