The extension will be tacked on to Smith's current deal, meaning he will be under contract with the team for the next 10 seasons. ESPN's Todd Archer and Adam Schefter report that the deal comes with $40 million guaranteed and a $10 million signing bonus.
The agreement is the largest non-QB contract in Cowboys history.
The Cowboys took Smith with the No. 9 pick in the 2011 draft. Smith started 16 games at right tackle his rookie season, and the team moved him to the left side the next year. In his three seasons in the league, Smith has started 47 of 48 possible games. Last season, he made his first Pro Bowl.
The deal, which will have an average annual value of $12.2 million per season once it kicks in, would make Smith the league's highest-paid offensive tackle. Cleveland Browns tackle Joe Thomas, the current highest-paid tackle, makes an average annual salary of $11.5 million. -- Alex Hampl