Lancastria may refer to:

• Lancashire, a British county and former duchy.
• The English House of Lancaster and its supporters ("Lancastrians")
• RMS Lancastria, British navy ship sunk in 1940 with the loss of about 4000 lives.
• another name for the trilobite Changaspis
• British Rail Class 40 diesel locomotive D223, built by English Electric at Newton-le-Willows, Lancashire

• Relentless Reckless Forever (album)
• Relentless Retribution
• Relentless Software
• Relentless: The True Story of the Man Behind Rogers Communications
• Relentless (Yngwie Malmsteen album)
• Relents
• Rele River
• Re (Les Rita Mitsouko album)
• Relessey, Ontario
• Relestat
• Reletively prime
• Relevance diagrams
• Relevance Language
• Relevance Vector Machine
• Relevant alternatives
• Relevant and appropriate requirements
• Relevant conduct
• Relevant cost
• Relevant (disambiguation)
• Relevant Podcast
• Relevos Suicida
• Relexified
• Relgions in prehistory
• Relhania
• Rel homology domain