The NSPCC said that the monitoring of sex offenders in communities is a national issue which needs “urgent attention”.
West Yorkshire Police says it has a dedicated team working on bringing the offenders to justice but the figures show one has been missing for nearly four years.
Registered sex offenders are required to inform the authorities of their address so they can be monitored.
West Yorkshire Police said the 13 are registered sex offenders who are currently wanted for breaching their registration requirements - but the force declined our request to identify the missing offenders.
Since 2000, there have been 430 cases of sex offenders going missing across the county.
Of those, most were found within nine days but 25 took over a year to find and two were only tracked down after a six-and-a-half-year hunt.
An NSPCC spokesman said: “The proven reoffending rate for convicted child sex offenders is around 1 in 10 so 2,000 plus will go on to commit further offences against children.
“Many crimes will go undetected so this is inevitably just the tip of the iceberg.
“The monitoring of registered sex offenders in communities needs urgent attention. Some of these offenders have committed the most serious of sexual offences against children but even the risk posed by offenders classed as lower risk can increase quickly, because of dynamic factors such as a relationship breakdown or a relapse into substance misuse.
“The effective monitoring of sex offenders in the community is a critical part in the prevention of child sexual abuse.”
Det Ch Insp Sue Jenkinson, head of safeguarding, said: “Protecting the public from sexual and violent offenders is at the heart of everything we do.
“West Yorkshire Police have dedicated, specialist teams based within our policing Districts who are assigned specifically to the management of registered sex offenders. These teams include Detective Constables, Detective Sergeants, Public Protection Review Officers and other administrative roles performed by key civilian employees.
“The Force also has a central unit, again with a dedicated function in this area, whose tasks include guiding and advising these local teams wherever appropriate in respect of procedure or changing national good practice.
“These localised specialist teams work from within District based Safeguarding teams. A risk assessment, in line with national practice, is made on each offender to determine the potential risk level they may pose whilst at liberty within the community and this risk level is constantly reviewed to ensure its accuracy against evolving information.
“Specific police actions to minimise risk are driven by this risk assessment process. Every breach of the legislation is fully investigated and the offender brought to justice, whenever that is possible.
“This includes cases where an offender’s whereabouts are unknown.”