The new model, Fred the frog, took 244 hours to build with 75,000 LEGO bricks and was unveiled by school children on Friday.
The giant brick sculptures will be at the attraction in Lancashire for six weeks this summer from July 16 to September 4, alongside some of their living counterparts.
Visitors on the trail will be able to enjoy a variety of brick LEGO characters inspired by real species at the wetland nature reserve.
The trail is one of eight activities Martin Mere is hosting this summer as part of its Nature Explorer attraction.
Nick Brooks, Martin Mere General Manager, said: “We are very excited to welcome back these amazing sculptures to WWT Martin Mere for the summer. The characters look fantastic, especially the new addition: Fred the Frog, our visitors were bowled over by them when we had these last Christmas.
“Being a Nature Explorer at Martin Mere is going to be great fun for all the family and a fantastic way to inspire kids to love wildlife.”