X40 may refer to:

• Boeing X-40, test platform for the X-37 Future-X Reusable Launch Vehicle
• SJ X40, series of electric multiple units operated by SJ of Sweden
• ThinkPad X40, laptop computer by Lenovo

• Chatham Island Oystercatcher
• Chatham Islands (British Columbia)
• Chatham Islands Duck
• Chatham Islands Gerygone
• Chatham Island Shag
• Chatham Islands Kaka
• Chatham Islands skink
• Chatham islands skink
• Chatham Island Standard Time
• Chatham Islands Territory
• Chatham Island temperate forests
• Chatham Island Warbler
• Chatham Island (Western Australia)
• Chatham Isles
• Chatham Jail
• Chatham Journal
• Chatham kaka
• Chatham--Kent
• Chatham, Kent, England
• Chatham--Kent--Essex
• Chatham-Kent--Essex
• Chatham-Kent--Essex (provincial electoral district)
• Chatham-Kent, Ontario
• Chatham Light Station
• Chatham Line