Testicular Cancer (Germ Cell Tumor): This is the most common diagnosis for a solid testicular mass in adults, with the majority being germ cell tumors such as seminoma or non-seminoma.

Epididymitis or Epididymal Tumor: Inflammation or tumors of the epididymis can present as a solid mass adjacent to the testis and may be difficult to distinguish from testicular pathology without imaging.
Testicular Lymphoma: Although more common in older men, lymphoma can present as a solid testicular mass and is an important consideration in the differential diagnosis.
Metastatic Disease: Metastases to the testis from other primary sites, such as the prostate, lung, or melanoma, can occur, though they are less common.

Testicular Torsion: Although typically presenting with acute pain, a testicular torsion can sometimes present with a painless mass if the torsion is intermittent or if there is a delay in seeking medical attention. Missing this diagnosis can result in loss of the testicle.
Intratesticular Abscess: An abscess within the testis can present as a solid mass and requires prompt antibiotic treatment and possibly drainage to prevent complications.

Adenomatoid Tumor: A rare, benign tumor of the testis or epididymis that can present as a solid mass.
Cystic Testicular Lesions: Such as simple cysts or epidermoid cysts, which are benign but can be confused with solid masses on physical examination.
Testicular Leiomyoma: A rare, benign smooth muscle tumor that can occur in the testis.
Sertoli Cell Tumor: A rare sex cord-stromal tumor that can present as a testicular mass.