The large size of CRISPR-Cas enzymes limits their delivery for therapeutic applications. Cas12j nucleases offers hypercompact alternative but show moderate editing efficiency. To overcome this limitation, we identified eight novel Cas12j orthologues (Cas12j-11 to Cas12j-18) from viral metagenomes. All showed low editing activity in mammalian cells. We engineered T5 exonuclease-Cas12j fusions (T5Exo-Cas12j), two of which, T5Exo-Cas12j-12, and -18 exhibited up to 42% editing in HEK293T and 9% in K-562 cells, outperforming wild-type Cas12j counterparts and comparable to LbCas12a. Intriguingly, robust in cellula editing in both HEK293T and K-562 cells was strictly dependent on the presence of 5′-TAC trinucleotides within the target DNA sequence. Furthermore, we fused the Cas12j orthologues with the TadA8e deaminase and developed base editors, termed Be-(d)Cas12j. Among these, Be-(d)Cas12j-13 demonstrated efficient A-to-G base conversion in mammalian cells. This study expands the CRISPR toolbox by characterizing and engineering novel Cas12j orthologues into compact, high-efficiency genome editors.