Its stock jumped more than 26 percent in premarket trading.
Williams said ETE's bid significantly undervalues the business, but that it was exploring other strategic options.
ETE confirmed Monday that it had offered $64 per share, a 32 percent premium to Williams' closing price Friday. It put the deal's total value at $53.1 billion, including debt and other liabilities.
Energy Transfer Equity LP, of Dallas, says it's made multiple attempts over the last six months to negotiate with Williams' senior management.
Energy experts have been predicting more consolidation in the energy transportation sector, especially in natural gas given the huge reserves only recently unlocked by new drilling methods.
The deal would give ETE access to Williams' assets in the northeast. Most of ETE's operations are in the Midwest and South.
Williams, based in Tulsa, Oklahoma, recently announced plans to acquire the remaining stake of Williams Partners LP that it doesn't already own. ETE's offer was contingent on Williams endings its plans to acquire Williams Partners.