According to food science expert and author Harold McGee: no, searing meat doesn't seal in moisture.

Food Network's Alton Brown, who agrees with McGee, even conducted an experiment to see if searing helps keep meat moist. Brown measured the moisture content of both seared and un-seared meat that had been cooked to the same internal temperature. The result: seared meat actually had less moisture.

We still think that searing is the way to go. Properly searing meat gives it a brown crust, which lends two things to the finished dish: flavor and
contrast. The rich flavor and mouthfeel of a beautifully browned crust makes the interior seem juicier (even if it isn't). 

So while searing may remove more moisture on balance, we think the difference is negligible, especially given the added flavor and contrast of a browned exterior.