Abstract: Provided is a lid having a lid body and multiple ribs. The lid body has an extending portion and a connecting portion connected with the extending portion. The extending portion is formed with an opening at one end distal to the connecting portion. The connecting portion protrudes outwardly along the lid's radial direction, and one end of the connecting portion distal to the extending portion dents inwardly to form an annular containing space. Multiple ribs are formed protruding on where the extending portion is adjacent to the connecting portion. Also provided is a container assembly including the lid and a cup body having a cup rim. With one end of the connecting portion distal to the extending portion continuously denting toward inside of the lid body, the cup rim and the connecting portion can be tightly engaged.