The companies announced on Wednesday that the deal was valued at $17.3 billion, including debt.
The offer of $305.39 per share represents a premium of about 32 percent to WellCare's closing price on Tuesday.
Shares of WellCare rose 23 percent to $284 in premarket trading.
The deal will create a premier healthcare enterprise focused on government-sponsored healthcare programs and a leader in Medicaid, Medicare and the Health Insurance Marketplace.
"This transformational combination creates a leading healthcare enterprise that is committed to helping people live healthier lives through a localized approach and provides access to high-quality healthcare through a wide range of affordable health solutions," said Michael F. Neidorff, Centene's Chairman and Chief Executive Officer. "With the addition of WellCare, we expect to bolster and diversify our product offerings, increase our scale and have access to new markets, which will in turn, enable us to continue investing in technology and better serve members with innovative programs designed to meet their needs."
After the deal, the company would have about 22 million members in the United States.