The fowl adenovirus field strain FAdV-JSN-5/10j (GenBank accession number KP879219) was isolated from the intestine of a 7-week-old chicken diagnosed with inclusion body hepatitis and simultaneously with Marek’s disease, and for that reason, it was chosen for molecular study. It was identified as fowl adenovirus genotype 7 (species Fowl aviadenovirus E) based on nucleotide sequence analysis of the loop L1 region of the hexon gene. Nucleotide sequence alignment of this strain, FAdV-7 reference strains B-3A ATCC VR-832 (AF339922) and YR36 (AF508955), and eight additional FAdV-7 field strains confirmed its classification as FAdV-JS-5/10j and showed that these viruses are very similar to each other. Additionally, we described mutations and their influence on the amino acid sequence, nucleotide composition, and relative synonymous codon usage. Immunofluorescence of cell cultures infected with 104.5 TCID 50 per 0.1-ml dose of the FAdV-JSN-5/10j strain demonstrated the presence of a cytopathic effect. Infection of fowl with adenoviruses raises concerns for poultry production, and thus, the efficient detection of adenovirus infection is crucial. This is the first attempt to describe the molecular characteristics of FadV-7 strains isolated in Poland.