Ensco has acquired Atwood Oceanics for $839 million.
Atwood shareholders will receive 1.6 Ensco shares for each Atwood share.
Both companies are offshore drilling contractors and the combination creates a group with 37 offshore oil rigs.
The merger could be a sign of future consolidation in an industry which has been hit hard in recent years by low oil prices and a lack of new oilfield drilling locations.
Morgan Stanley, DNB Markets and HSBC Securities were Ensco's financial advisers.