Salwa is a kingdom grouped among the western kingdoms in the epic Mahabharata. It is close to Madra Kingdom as both are mentioned together in many places. Saubha is its capital. Martikavati (alias Matika, Matrika, Matrikavati) also is mentioned as the capital of Salwa kingdom. The famous prince Satyavan was from Salwa. He married the Madra princess, Savitri, the daughter of Madra king Aswapati. Their history is a famous narration in Mahabharata. Seven chapters 3:291 to 3:297 is dedicated to this history of Satyavan and Savitri.
References
Salwa Kingdom Wikipedia(Text) CC BY-SA