Abstract Background Staphylococcal cassette chromosome mec (SCCmec) elements are highly diverse and have been classified into 13 types. The arginine catabolic mobile element (ACME) is an SCC-like element harbouring an arginine deiminase pathway gene cluster (ACME-arc). ACME type I (ACME I), additionally including a spermidine/spermine-N1-acetyltransferase gene (speG), is considered to have contributed to the rapid spread of the most successful MRSA clone, USA300. Objectives To characterize the SCC composite islands (SCC-CIs) in ST5 MRSA positive for both ACME-arc and speG. Methods Three ST5 MRSA strains (SC640, SC792 and SC955) collected in Hokkaido, Japan were subjected to WGS and the SCC-CIs were determined. Results The SCC-CIs consisted of four (SC640 and SC792) or three (SC955) SCC/SCC-like elements and commonly harboured both an ACME type II′ and an SCC encoding speG. These SCC-CIs appear to mimic ACME I in USA300, in that they are equipped with ACME-arc and speG. The SCC-CIs of SC640 and SC792 contained novel SCCmec/SCCmec-like elements at the 3′ end, whereas SC955 contained SCCmec type V. The SCCmec of SC792 carried mec complex A and ccrC1, which was determined to be novel and designated as SCCmec type XIV (5A). SC640 harboured an SCCmec-like element derived from SCCmec type XIV. It lacked most of the downstream region of the mec complex, including the left chromosomal attachment site (SCCmec XIV Δkdp/DR-L), and lost its capability for chromosomal excision, suggesting that the mecA gene is immobilized on the chromosome. Conclusions These findings provide evidence for increasing complexity of SCC-CIs.