CINCINNATI — Offensive tackle Andre Smith signed a contract Sunday with the Cincinnati Bengals, ending an impasse that dragged on a month into the preseason.

The Bengals originally planned to have Smith start at right tackle, but he has missed three preseason games. He was in Cincinnati and agreed to the deal shortly before the start of a late afternoon practice, allowing him to get back on the field.

The Bengals finish their preseason schedule with a home game against Indianapolis on Thursday. It’s unclear whether the rookie will be ready to play in that game.

‘‘We’ll ease him into practice, see where his conditioning level is and see where he is this week,’’ coach Marvin Lewis said. ‘‘We haven’t made a decision about Thursday and won’t know until after Wednesday.’’

Smith, the sixth overall pick, was the highest unsigned NFL pick with one week left in the preseason. The Bengals had insisted on paying him less than a player taken later in the draft, Oakland’s Darrius Heyward-Bey, leading to the impasse.

Smith’s contract leaves Michael Crabtree, drafted by San Francisco with the 10th overall pick, as the only unsigned first-round selection.

The four-year deal includes $21 million guaranteed. The team has an option after the 2010 season that would turn it into a six-year deal that includes $29.5 million guaranteed. Heyward-Bey got a five-year deal with $23.5 million guaranteed from the Raiders.

Smith participated in the afternoon practice, doing some drills with other linemen during the portion of the workout open to the media.

‘‘It was difficult just sitting there waiting. I’m eager to get back to football,’’ Smith said after practice. ‘‘I don’t feel like I’m that far behind. There’s a difference between workout shape and football shape. I’m ready to get back into football shape.’’

The team gave Smith’s bed to another player during training camp in Georgetown, Ky. HBO’s ‘‘Hard Knocks’’ episode last Wednesday showed teammates mocking Smith’s weight during the team’s annual rookie skits.

Prolonged holdouts are commonplace in Cincinnati. Last year, linebacker Keith Rivers missed nine days of camp and was one of the last two first-round picks to come to terms.

The Bengals’ offensive line was one of their top priorities in the offseason. The offense finished last in the league in 2008 in large part because of the line’s ineffectiveness.

Quarterback Carson Palmer missed a dozen games because he tore a ligament and tendon in his throwing elbow when he got hit while passing.

Only right guard Bobbie Williams returns in the same spot this year. The Bengals let their two tackles leave and drafted Smith, whose holdout forced them to revamp their plans. Anthony Collins has played right tackle in Smith’s absence.

The Bengals open the season at home against the Denver Broncos. Lewis has said throughout training camp that Smith will get the chance to compete for the starting job at right tackle once he gets into camp.

The Bengals also are unsure whether Palmer will play in the final preseason game. He has missed the last two games with a moderately sprained left ankle. The Bengals don’t want him to aggravate the injury, which would jeopardize his chances of playing against the Broncos.