Batth, the 21-year-old defender who has extended his stay from Wolves until the end of the season, headed in Chris Lines' cross after a short corner caught the struggling Lilywhites napping after four minutes.
Marshall, who Owls boss Gary Megson wanted to sign permanently before he agreed a new deal with Stoke, then added a second for Wednesday, while Jamie Douglas and Paul Parry wasted second-half chances for Preston.
After Batth nodded Wednesday in front Preston were fortunate to survive the next five minutes unscathed as Wednesday won five corners in quick succession.
Ryan Lowe should have doubled the visitors' lead in the 14th minute but the in-form forward was thwarted by Thorsten Stuckmann's legs when through on goal.
Stuckmann could not keep the visitors out for long, however, and Marshall beat him from an acute angle after 42 minutes to double the Owls' advantage. Preston sub Douglas saw his effort cleared off the line by Miguel Llera after 79 minutes before Parry headed wide from six yards with five minutes to go.